Comparison of different SQL implementations(整理)
Comparison of different SQL implementations
The goal of this page — which is a work in progress — is to gather information relevant for people who are porting SQL from one product to another and/or are interested in possibilities and limits of 'cross-product' SQL.
The following tables compare how different DBMS products handle various SQL (and related) features. If possible, the tables also state how the implementations should do things, according to the SQL standard.
I will only write about subjects that I've worked with personally, or subjects which I anticipate to find use for in the near future. Subjects on which there are no significant implementation variances are not covered. Beta-versions of software are not examined.
I'm sorry about the colors. They are a result of wanting to mark each DBMS differently and at the same time wanting to be relatively nice to printers.
If you have corrections or suggestions, please contact me; even notifications about spelling errors are welcome.
Contents:
- Legend, definitions, and notes
- Features
- Data definition language (DDL)
- The SELECT statement
- Ordering result sets
- Limiting result sets
(RANK() / ROW_NUMBER() / FETCH FIRST / LIMIT / TOP)
- Simple limit
- Top-n (quota-queries)
- Limit—with offset , including a note about the importance of sorting on unique values
- The INSERT statement
- Data types
- Functions and operators
- Constraint handling
- Mixture of type and operations
- Automatic key generation (IDENTITY/SERIAL/AUTO_INCREMENT)
- Bulk operations
- Command line operations / metadata
- JDBC
- Other topics
- Related work
- Acknowledgments
- TODOs
Legend, definitions, and notes
The following SQL standard and implementations have been examined, if not otherwise stated:
Standard | The latest official version of SQL is SQL:2008.
I don't have access to the official ISO standard text, but
Whitemarsh Information Systems Corporation
provides a rather final draft
as a zip-archive, containing several files. Most important to this
page is the file No books cover SQL:2008 yet. Regarding the previous standard, SQL:2003, the only book covering the subject
is in German which I was never any good at.
Therefore, I also use the following book as reference:
|
PostgreSQL |
PostgreSQL 8.4.1 on CentOS Linux. Documentation |
DB2 |
DB2 Express-C v. 9.1 on Fedora Linux. Note that there are differences between various DB2 flavors
; this page is about DB2 for "LUW" (Linux/Unix/Windows). Documentation |
MS SQL Server |
MS SQL Server 2005 on Windows XP.
Microsoft's SQL implementation is sometimes named Transact-SQL
, or TSQL
.
In this document, I'll generally write MSSQL
as a short-hand for
Microsoft's SQL Server product. Documentation |
MySQL |
MySQL Database Server 5.0.18 on Fedora Linux (i.e. MySQL AB's "classic" DBMS product—not MaxDB). Documentation |
Oracle |
Oracle Database 11g
Release 2 on Red Hat Enterprise Linux. Documentation |
Informix |
Informix Dynamic Server Workgroup Edition v. 11.50 on Red Hat Enterprise Linux. Documentation |
The products are running with their default settings. This is important for MySQL and MSSQL: Their interpretation of SQL may be changed rather drastically by adjusting certain configuration options, potentially increasing the level of standard compliance (for MySQL, there is a dedicated documentation page about this). However, such non-default configuration options are not of great value for people writing SQL applications because the developer often cannot rely on non-default configuration settings.
Features
Views
Standard | Views are part of the standard, and they may be updated, as long as it 'makes sense'.
SQL:2008 has a rather complicated set of rules governing when a view is updatable, basically saying that a view is updatable, as long as the update-operation translates into an unambiguous change. SQL-92 was more restrictive, specifying that updatable views cannot be derived from more than one base table. |
PostgreSQL | Has views. Breaks that standard by not allowing updates to views; offers the non-standard 'rules'-system as a work-around. |
DB2 | Conforms to at least SQL-92. |
MSSQL | Conforms to at least SQL-92. |
MySQL | Conforms to at least SQL-92. |
Oracle | Conforms to at least SQL-92. |
Informix | Conforms to at least SQL-92. |
Join types and features
All the DBMSes support basic INNER JOINs, but vary in their support for other join types.
In the following feature chart, a means yes ; an empty table cell means no .
Join type/feature | PostgreSQL | DB2 | MSSQL | MySQL | Oracle | Informix |
---|---|---|---|---|---|---|
Natural joins (only tested: NATURAL LEFT JOIN
) |
||||||
USING
-clause |
||||||
FULL joins1
(tested: SELECT...FULL JOIN...ON...=...
) |
||||||
Explicit CROSS JOIN
(cartesian product) |
Remarks:
- Note that
FULL
joins may be emulated with a union of a left and a right join .
Data definition language (DDL)
Copying structure
Objective: An existing table, t1 needs to be copied to a new table, t2 , without copying data. I.e., only the structure/definition of the table is copied.
Standard | Optional feature T171 defines LIKE clause in table definition
:
CREATE TABLE t2
( LIKE t1
)
The DBMS may support an extension of this (feature T173) which allows for more table properties to be copied:
If Triggers, CHECK constraints, and other 'non-trivial' table features are not copied to the new table. |
PostgreSQL | Complies
with the core of the feature (T171). The extended T173 feature is only
partially supported, and extended with a few non-standard options:
PostgreSQL does not allow you to copy the structure of a view, using |
DB2 | Behaves as if inspired
by the standard. I.e., DB2 conforms to the standard, except:
Example:
DB2 allows you to copy the structure of a view into a table. |
MSSQL | Does not support the standard. Instead, MSSQL has a special SELECT ... INTO ... FROM ...
construct which can be combined with an impossible WHERE-clause to copy structure only:
SELECT * INTO t2
FROM t1
WHERE 1<>1
The source (t1 ) may be a view, as well as a table.
|
MySQL | Complies with the core of the feature (T171), but not with the extended features (T173).
MySQL does not allow you to copy the structure of a view into a table. |
Oracle | Does not support the standard. Oracle lets you copy a table structure using a special CREATE TABLE ... AS
construct, combined with an impossible WHERE
-clause:
CREATE TABLE t2
AS SELECT * FROM t1
WHERE 1<>1
|
Informix | On my TODO. |
The SELECT statement
Ordering result sets
Standard | The SQL-standard states that relations are unordered, but
result sets may be ordered when returned to the user through a cursor:
The DBMS may additionally allow The
standard doesn't
specify how NULLs should be ordered in comparison with
non-NULL values, except that any two NULLs are to be considered equally ordered, and
that NULLs should sort either above or below all non-NULL values. However, the DBMS may
optionally (as part of feature ID T611, "Elementary OLAP operations")
allow the user to specify whether NULLs should sort first or last:
|
PostgreSQL | As well as in cursor definitions, it allows ORDER BY
in other contexts.
By default, NULLs are considered higher
than any non-NULL value; however,(since version 8.3)
this sorting behaviour may be changed by adding |
DB2 | As well as in cursor definitions, it allows ORDER BY
in other contexts.
NULLs are considered higher
than any non-NULL value.
|
MSSQL | As well as in cursor definitions, it allows ORDER BY
in other contexts.
NULLs are considered lower
than any non-NULL value.
|
MySQL | As well as in cursor definitions, it allows ORDER BY
in other contexts.
NULLs are considered lower
than any non-NULL value,
except if a |
Oracle | As well as in cursor definitions, it allows ORDER BY
in other contexts.
By default, NULLs are considered higher
than any non-NULL value; however, this sorting behaviour may be changed
by adding Beware of Oracle's strange treatment of empty strings and NULLs as the same 'value'. |
Informix | As well as in cursor definitions, it allows ORDER BY
in other contexts.
NULLs are considered lower
than any non-NULL value.
|
Limiting result sets
Simple limit
Objective: Want to only get n
rows in the result set.
Usually only makes sense in connection with an ORDER BY
expression.
Note: This is not the same as a top-n query — see next section .
Note also: Some of the queries below may not be legal in all situations, such as in views or sub-queries.
Standard | The SQL standard provides three ways of performing a 'simple limit':
|
PostgreSQL |
Supports all standards-based approaches.
In old PostgreSQL versions (versions 8.3 and older), a special PostgreSQL (and MySQL) specific method was used:
Note that Documentation: |
DB2 | Supports all standards-based approaches.
Documentation:
|
MSSQL |
Supports the ROW_NUMBER()
(since MSSQL 2005)
and cursor standards-based approaches; doesn't support FETCH FIRST
.
MSSQL 2000 didn't support |
MySQL |
Doesn't support the standard. Alternative solution:
|
Oracle |
Supports ROW_NUMBER
; doesn't support FETCH FIRST
.
As Oracle doesn't allow
A reader of this page told me that using the Oracle-specific |
Informix | Doesn't support ROW_NUMBER(), nor FETCH FIRST.
Alternative solution (which is illegal in plain sub-queries):
|
Top-n query
Objective: Like the simple limit-query above, but include rows with tie conditions. Thus, the query may return more than n rows.
Some call this a quota -query.
The following examples are based on this table:
SELECT * FROM person ORDER BY age ASC;
+----------+-------------+-----+
|PERSON_ID | PERSON_NAME | AGE |
+----------+-------------+-----+
| 7 | Hilda | 12 |
| 8 | Bill | 12 |
| 4 | Joe | 23 |
| 2 | Veronica | 23 |
| 3 | Michael | 27 |
| 9 | Marianne | 27 |
| 1 | Ben | 50 |
| 10 | Michelle | 50 |
| 5 | Irene | 77 |
| 6 | Vivian | 77 |
+----------+-------------+-----+
Now, we only want the three (n =3) youngest persons displayed, i.e. a result set like this:
+----------+-------------+-----+
|PERSON_ID | PERSON_NAME | AGE |
+----------+-------------+-----+
| 7 | Hilda | 12 |
| 8 | Bill | 12 |
| 4 | Joe | 23 |
| 2 | Veronica | 23 |
+----------+-------------+-----+
Standard | With standard SQL, there are two principal ways to obtain the wanted data:
In the article Going To Extremes by Joe Celko , there is a description of yet another principle for performing quota queries, using scalar subqueries . Scalar subqueries are more tedious to write but might yield better performance on your system. |
PostgreSQL |
Supports the fast standard SQL
variant.
In version 8.3 and older, PostgreSQL only supported the slow standard SQL
query variant. In practice, a PostgreSQL-only method was used instead, in order to obtain
acceptable query performance: (Change |
DB2 | Supports the fast standard SQL variant. |
MSSQL |
Supports the fast standard SQL
variant.
MSSQL 2000 supported the slow standard SQL
variant. In practice, a MSSQL-only expression had to be used instead, in order to obtain acceptable query performance:
|
MySQL |
Supports the slow standard SQL
solution. In practice, this MySQL-specific solution should be used instead,
in order to obtain acceptable query performance:
(Change The offset-value 2 is the result of n-1 (remember: n is 3 in these examples). The second argument to the |
Oracle |
Supports the fast standard SQL
variant.
However, as Oracle doesn't like "AS ...
" after subqueries
(and doesn't require naming of subqueries), the query has to be
paraphrased slightly:
(Change |
Informix | On my TODO. |
Limit—with offset
Objective: Want to only get n
rows in the result set,
and we want the first skip
rows in the result set discarded.
Usually only makes sense in connection with an ORDER BY
expression.
In the recipes below, basic ordering is ASCending, i.e.
lowest-first queries. If you want the opposite, then change
ASC->DESC
and DESC->ASC
at the places emphasized like this
.
Standard | The SQL standard provides three ways of performing 'limit with offset':
|
PostgreSQL |
Supports all the standards-based approaches.
In version 8.3 and older, cursors should be used, or a special construct: Documentation: |
DB2 | Supports the window function
based approach.
Regarding cursors: DB2 for Linux/Unix/Windows doesn't support Documentation : OLAP functions , the FETCH statement . |
MSSQL |
Supports the window function
and cursor based approaches.
MSSQL 2000 didn't support |
MySQL |
Doesn't support the standard approaches. Alternative solution:
SELECT columns
In older versions of MySQL, the LIMIT-syntax is less clear:
|
Oracle |
Supports ROW_NUMBER()
. I'm unsure if Oracle's cursor support is standards-compliant.
As Oracle doesn't accept
A reader of this page told me that using the Oracle-specific |
Informix | Supports neither OFFSET
...FETCH FIRST
nor ROW_NUMBER
. Supports cursors.
An alternative to using cursors is to us an Informix-specific construct:
|
FETCH FIRST/LIMIT/TOP queries with offset are often used in a result presentation context: To retrieve only—say—30 rows at a time so that the end-user isn't overwhelmed by the complete result set, but instead is offered a paginated result presentation. In this case, be careful not to (only) sort on a non-unique column.
Consider the following example (where PostgreSQL is used):
SELECT * FROM person ORDER BY age ASC;
person_id | person_name | age
-----------+-------------+-----
7 | Hilda | 12
8 | Bill | 12
4 | Joe | 23
2 | Veronica | 23
3 | Michael | 27
9 | Marianne | 27
1 | Ben | 50
10 | Michelle | 50
5 | Irene | 77
6 | Vivian | 77
When ordering is performed on the non-unique age-value, ties may occur and it's not guaranteed that the DBMS will fetch the rows in the same order every time.
Instead of the above listing, the DBMS is allowed to return the following display order where Michael and Marianne are displayed in the opposite order compared to above:
SELECT * FROM person ORDER BY age ASC;
person_id | person_name | age
-----------+-------------+-----
7 | Hilda | 12
8 | Bill | 12
4 | Joe | 23
2 | Veronica | 23
9 | Marianne | 27
3 | Michael | 27
1 | Ben | 50
10 | Michelle | 50
5 | Irene | 77
6 | Vivian | 77
Now, suppose the end-user wants the results displayed five rows at a time. The result set is fetched in two queries where the DBMS happens to sort differently, as above. We will use PostgreSQL's legacy syntax in the example:
SELECT * FROM person ORDER BY age ASC LIMIT 5;
person_id | person_name | age
-----------+-------------+-----
7 | Hilda | 12
8 | Bill | 12
4 | Joe | 23
2 | Veronica | 23
3 | Michael | 27
SELECT * FROM person ORDER BY age ASC LIMIT 5 OFFSET 5;
person_id | person_name | age
-----------+-------------+-----
3 | Michael | 27
1 | Ben | 50
10 | Michelle | 50
5 | Irene | 77
6 | Vivian | 77
Notice that Marianne was not displayed in any of the two split result set presentations.
The problem could be avoided if the result set ordering had been done in
a deterministic way, i.e. where the unique person_id value was considered
in case of a tie:
SELECT * FROM person ORDER BY age ASC, person_id ASC ...
This is safer than to pray for the DBMS to behave in a predictable way when
handling non-unique values.
Note : If the table is updated between parts of the result set pagination, then the user might still get an inconsistent presentation. If you want to guard against this, too, then you should see if use of an insensitive cursor is an option in your application. Use of cursors to paginate result sets usually require that your application is stateful , which is not the case in many web-application settings. Alternatively, you could let the application cache the complete result set (e.g. in a session if your web application environment provides for sessions).
The INSERT statement
Inserting several rows at a time
Standard | An optional SQL feature is row value constructors
(feature ID F641). One handy use
of row value constructors is when inserting several rows at a time, such as:
— which may be read as a shorthand for
|
PostgreSQL | Supported .(since version 8.2) |
DB2 | Supported . |
MSSQL | Supported .(since version 2008) |
MySQL | Supported . |
Oracle | An Oracle-specific kludge:INSERT INTO tablename
|
Informix | On my TODO. |
Data types
The BOOLEAN type
Standard | The BOOLEAN type is optional (has feature ID T031), which is
a bit surprising for such a basic type. However, it seems that endless discussions of how
NULL is to be interpreted for a boolean value is holding BOOLEAN from becoming
a core type.
The standard says that a BOOLEAN may be one of the following literals:
The DBMS may interpret NULL as equivalent to UNKNOWN. It is unclear from the specification if the DBMS must support UNKNOWN, NULL or both as boolean literals. In this author's opinion, you should forget about the UNKNOWN literal in order to simplify the situation and let the normal SQL three-way logic apply. It's defined that TRUE > FALSE (true larger than false). |
PostgreSQL | Follows the standard.
Accepts NULL as a boolean literal; doesn't accept UNKNOWN as a boolean literal. |
DB2 | Doesn't support the BOOLEAN type.
Judging from various JDBC-documentation, it seems that IBM recommends a CHAR(1) field constrained to values '0' and '1' (and perhaps NULL) as the way to store boolean values. |
MSSQL | Doesn't support the BOOLEAN type.
Possible alternative type: the BIT type which may have 0 or 1 (or NULL) as value. If you insert an integer value other than these into a field of type BIT, then the inserted value will silently be converted to 1. Rudy Limeback has some notes about oddities with the MSSQL BIT type. |
MySQL | Offers a non-conforming BOOLEAN type. MySQL's BOOLEAN
is one of many aliases to its TINYINT(1) type.
MySQL accepts the literals TRUE and FALSE as aliases to 1 and 0, respectively. However, you may also assign a value of — e.g. — 9 to a column of type BOOLEAN (which is non-conforming). If you use JDBC with MySQL, then BOOLEAN is the preferred type for booleans: MySQL's JDBC-driver implicitly converts between Java's boolean and MySQL's pseudo-BOOLEAN type. Side note: MySQL has a |
Oracle | Doesn't support the BOOLEAN type.
Judging from various JDBC documentation and a discussion at Ask Tom , it seems that Oracle recommends NUMBER(1) as the way to store boolean values; it's probably wise to constrain such columns to values 0 and 1 (and perhaps NULL). |
Informix | On my TODO. |
Warning
to JDBC users:
According to the JDBC standard, getBoolean()
must convert a
SQL-'value' of NULL to the false
Java value. To check if
the database-value was really NULL, use wasNull()
.
The CHAR type
For the following section, I have used this test-SQL to try to illuminate differences (unfortunately, even standard SQL as simple as this has to be adjusted for some products):
Test steps:
CREATE TABLE chartest (
charval1 CHAR(10) NOT NULL,
charval2 CHAR(10) NOT NULL,
varcharval VARCHAR(30) NOT NULL
);
INSERT INTO chartest VALUES ('aaa','aaa','aaa');
INSERT INTO chartest
VALUES ('aaaaaa ','aaa','aaa'); -- should truncate to 'aaaaaa '
INSERT INTO chartest
VALUES ('aaaaaaaaaaaa','aaa','aaa'); -- should raise error
SELECT * FROM chartest; -- should show two rows
DELETE FROM chartest WHERE charval1='aaaaaa';
SELECT * FROM chartest; -- should show one row
SELECT * FROM chartest WHERE charval1=varcharval;
SELECT charval1 || 'X' AS res FROM chartest;
SELECT CHAR_LENGTH(charval1 || charval2) AS res FROM chartest;
SELECT CHAR_LENGTH(charval1) + CHAR_LENGTH(charval2)
AS res
FROM chartest;
Expected results, after CREATE and INSERTs:
SELECT * FROM chartest; -- should show two rows
CHARVAL1 CHARVAL2 VARCHARVAL
========== ========== ==============================
aaa aaa aaa
aaaaaa aaa aaa
DELETE FROM chartest WHERE charval1='aaaaaa';
SELECT * FROM chartest; -- should show one row
CHARVAL1 CHARVAL2 VARCHARVAL
========== ========== ==============================
aaa aaa aaa
SELECT * FROM chartest WHERE charval1=varcharval;
CHARVAL1 CHARVAL2 VARCHARVAL
========== ========== ==============================
aaa aaa aaa
SELECT charval1 || 'X' FROM chartest AS res;
res
===========
aaa X
SELECT CHAR_LENGTH(charval1 || charval2) AS res FROM chartest;
res
===========
20
SELECT character_length(charval1) + character_length(charval2)
AS res
FROM chartest;
res
============
20
Standard |
|
PostgreSQL | Stores CHARs in space padded form, but violates the standard by (conceptually) truncating
trailing white-space before performing most functions, operators, and comparisons
(like the CHARACTER_LENGTH
-function
and the
concatenation(||
) operator).
|
DB2 | Follows the standard. |
MSSQL | Generally follows standard, but (conceptually) truncates
trailing white-space before performing some functions
(at least before LEN()
).
|
MySQL | Breaks the standard by silently inserting the string, truncated to specified column CHAR-length. (It's actually not completely silent, as it issues warnings if values were truncated: If you manually check for warnings, you will know that something bad happened, but not which of the rows are now invalid.) Violates the standard by effectively truncating all trailing spaces. The documentation states that MySQL truncates trailing spaces when CHAR values are retrieved . That may be true, but it seems that truncation even happens before the CHAR values are used as input in functions like CONCAT
, CHAR_LENGTH
, etc.
|
Oracle | Follows the standard, with a minor exception: Oracle doesn't remove trailing spaces which exceed the specified CHAR length, but raises an exception. |
Informix | On my TODO. |
Date and time
The TIMESTAMP type
Standard |
Part of the Core requirements, feature ID F051-03. Stores year, month, day, hour, minute, second (with fractional seconds; default is 6 fractional digits). Extension to Core SQL (feature ID F411): TIMESTAMP WITH TIME ZONE which also stores the time zone. Examples of TIMESTAMP literals:
Examples of TIMESTAMP WITH TIME ZONE literals:
It's strange that TIMESTAMP WITH TIME ZONE literals
are not represented as, e.g.,
|
PostgreSQL |
Follows that standard with one exception: In some cases, TIMESTAMP '2003-08-23 01:02:03 +02:00'
is interpreted
as a TIMESTAMP WITHOUT
TIME ZONE
(discarding the '+02:00
' part)—not
as a TIMESTAMP WITH TIME ZONE value. The standard
may be illogical regarding this, but a standard is a standard...
Performs good sanity checks on inserted timestamp values; e.g. this will work: |
DB2 |
DB2 has the TIMESTAMP data type, but not the extended TIMESTAMP WITH TIME ZONE type. DB2 accepts TIMESTAMP literals like '2003-07-23 00:00:00'
, however
it doesn't accept the typed TIMESTAMP '2003-07-23 00:00:00'
variant.
Performs good sanity checks on inserted timestamp values; e.g. this will work: |
MSSQL |
Note that MSSQL's choice of words related to date and time is
confusing: In MSSQL's vocabulary, datetime
is a concrete data type,
whereas in the SQL standard, datetime is a general term covering the
DATE, TIME and TIMESTAMP types.
MSSQL has a strange pseudo-type called TIMESTAMP, but has deprecated it; don't use it in new code. The closest match to the SQL standard's TIMESTAMP type is DATETIME . This type stores the combination of date and time. It has a maximum of three fractional digits for seconds. Performs good sanity checks on inserted timestamp values; e.g. this will work: |
MySQL |
No matter what date/time data type chosen in MySQL, storage of fractional
seconds and time zones are not supported
(the TIME
type accepts time literals with fractional seconds, but discards the
fractional part when storing the value). You will have to invent your
own systems for such information. Note also, that MySQL's choice of words related to date and time is confusing: In MySQL's vocabulary, datetime is a concrete data type, whereas in the SQL standard, datetime is a general term covering the DATE, TIME and TIMESTAMP types. MySQL has a type called TIMESTAMP, but it is quite different from the standard TIMESTAMP: It's a 'magic' data type with side effects in that it's automatically updated to the current date and time if some criteria are fulfilled. MySQL has a type called DATETIME. Like MySQL's TIMESTAMP type, it stores a combination of date and time without fractional seconds. There are no side effects associated with the DATETIME type—which makes it the closest match to the SQL standard's TIMESTAMP type. By default, MySQL's sanity checks with regard to dates and time are (deliberately) poor. For example, MySQL accepts DATETIME values of '2003-02-29 00:05:00' and '2003-01-32 00:00:00'. Such values yield warnings (which you must check for if you want to be warned), but result in a value of zero being stored. |
Oracle |
Follows the standard. Oracle has both the TIMESTAMP and the extended
TIMESTAMP WITH TIME ZONE types.
A special gotcha applies, though: Oracle forbids columns of type TIMESTAMP WITH TIME ZONE as part of a unique key ; this includes primary and foreign keys. Timestamps without time zone (and Oracle's special TIMESTAMP WITH LOCAL TIME ZONE) are accepted. Performs good sanity checks on inserted timestamp values; e.g. this will work: |
Informix | On my TODO. |
SQL functions
CHARACTER_LENGTH
Standard | CHARACTER_LENGTH(argument
)
If the optional feature T061 is implemented, the function may be augmented with an indication of string unit : CHARACTER_LENGTH(argument
USING string-unit
)
string-unit may be UTF8
, UTF16
, UTF32
.
Returns NUMERIC. Returns NULL if the input is NULL. |
PostgreSQL |
Follows the standard, providing CHARACTER_LENGTH
(and CHAR_LENGTH
).
Note that PostgreSQL removes trailing (not leading) space from from CHAR values before counting. Note also that the behaviour of CHARACTER_LENGTH with regard to CHAR values has changed between versions 7.4 and 8.0 of PostgreSQL. |
DB2 |
Has a CHARACTER_LENGTH function, but it's non-compliant because it requires indication of string unit
, and db2's string units are different from the standard's.
Provides the Note that CHAR values are space-padded (like the standard
says they should be), so the length of
Documentation : CHARACTER_LENGTH and LENGTH |
MSSQL |
Doesn't have CHARACTER_LENGTH. Provides the LEN
and DATALENGTH
functions instead (the latter is especially valid for 'special' data types like the TEXT
type).Note that MSSQL's LEN
-function removes trailing (not leading) spaces from CHAR values before counting; MSSQL's DATALENGTH
doesn't discard spaces.
Documentation : LEN and DATALENGTH |
MySQL |
Provides CHARACTER_LENGTH. Aliases: CHAR_LENGTH, LENGTH. Note that MySQL removes trailing (not leading) spaces from CHAR values before counting. |
Oracle |
Doesn't have CHARACTER_LENGTH. Provides the LENGTH
function instead.
Behaves in strange ways if the input is the empty string or NULL, because of Oracles non-standard NULL handling (it considers NULL and the empty string identical 'values'). Note that CHAR values are space-padded (like the standard
says they should be), so the length of
|
Informix | On my TODO. |
SUBSTRING
Standard | The standard defines two variants of the SUBSTRING function:
|
PostgreSQL | PostgreSQL provides three SUBSTRING flavors:
|
DB2 | Provides (since version 9)
the SUBSTRING
function, but requires you to indicate string unit by appending "USING unit
".
The unit identifier may be CODEUNITS16
, CODEUNITS32
, or OCTETS
. CODEUNITS16
/CODEUNITS32
seem non-standard. The standard's CHARACTERS
unit isn't supported by DB2.
Example: SELECT SUBSTRING(somecolumn
FROM 3
USING OCTETS) FROM sometable
SELECT SUBSTRING(somecolumn
FROM 3
FOR 2
USING OCTETS) FROM sometable
For old DB2 versions, use the non-standard DB2 doesn't provide any built-in regular expression facilities at all (but you may manually add PCRE capabilities ). |
MSSQL | MSSQL has a SUBSTRING
function, but its syntax differs from that of the standard. The syntax is:
where start is an integer specifying the beginning of the string, and length is a non-negative integer indicating how many characters to return. MSSQL has no regular expression functionality. |
MySQL | MySQL
supports the standard's ordinary SUBSTRING function, with some twists
(see below). No regular expression based substring extraction is
supported. MySQL breaks the standard when negative values are used as either start-position or length:
|
Oracle | Doesn't provide the standard SUBSTRING function. Provides SUBSTR(input
,start-pos
[,length
])
instead (i.e. length
is optional).
Oracle provides a number of SUBSTR-variants (SUBSTRB, SUBSTRC, SUBSTR2, SUBSTR4, same syntax as for SUBSTR), mainly for handling various kinds of non-latin-only string-types. Oracle doesn't have support for string-extraction with the special SQL-style regular expressions. Instead, it has the REGEXP_SUBSTR function which offers string extraction, using POSIX-style regular expression pattern matching. Documentation : SUBSTR and REGEXP_SUBSTR . |
Informix | On my TODO. |
Note : If you find yourself using SUBSTRING in a WHERE-expression, then consider if LIKE could be used instead: The use of LIKE will typically make your DBMS try to use an index, whereas it will typically not try to do so in connection with functions.
REPLACE
REPLACE
means a string-function which searches a source string (haystack) for occurrences of
a string to be replaced (needle) and replaces it with a new string (replacement).
Standard | Not mentioned. May be obtained
through a combination of other functions (have a look at the
OVERLAY, POSITION and CHARACTER_LENGTH functions).
A de facto standard seems to have emerged with regard to REPLACE: REPLACE (haystack
:string,needle
:string,replacement
:string)
which means 'replace needle with replacement in the string haystack '. Replacement is done case-sensitively unless otherwise stated. The REPLACE function may be handy for correcting spelling errors (and other situations):
UPDATE tablename
|
PostgreSQL | Follows de facto
standard.
Documentation |
DB2 | Follows de facto
standard.
Documentation |
MSSQL | Follows de facto
standard with the exception that MSSQL by default works case in
sensitively.
Documentation |
MySQL | Follows de facto
standard.
MySQL even works case sensitively .1 Note that the REPLACE
-function is different from MySQL's
non-standard REPLACE INTO
expression.
Documentation |
Oracle | Follows de facto
standard.
Documentation |
Informix | On my TODO. |
Note 1
:
In this author's opinion, it's confusing that most (if not all)
string-related functions in MySQL work case sensitively
,
while MySQL's default
behaviour is to work case in
sensitively in
plain WHERE-clauses
involving string comparisons.
TRIM
Standard |
Core SQL feature ID E021-09:
TRIM(where
characters
FROM string_to_be_trimmed
)
where may be one of LEADING, TRAILING or BOTH—or omitted which implies BOTH. characters indicates what character(s) to remove from the head and/or tail of the string. It may be omitted which implies the value ' ' (space character). In other words, the shortest form is Trimming NULL returns NULL. |
PostgreSQL | Follows the standard. |
DB2 |
Follows the standard.(since version 9.1)
In db2 versions lower than 9.1, you only have: |
MSSQL |
Doesn't support the standard TRIM function.
Provides |
MySQL | Follows the standard. |
Oracle | Follows the standard with two exceptions:
|
Informix | On my TODO. |
LOCALTIMESTAMP
It's often important to get the value of current date and time. Below are the functions used to do that in the different implementations.
Standard |
The current timestamp (without time zone) is retrieved
with the LOCALTIMESTAMP function which may be used as:
Note that " If the DBMS supports the non-core time zone features (feature ID F411), then
it must also provide the functions |
PostgreSQL | Follows the standard. |
DB2 |
Doesn't have the LOCALTIMESTAMP function.
Instead, it provides a special, magic value ('special register' in IBM language), CURRENT_TIMESTAMP (alias to 'CURRENT TIMESTAMP') which may be used as though it were a function without arguments. However, since DB2 doesn't provide TIMESTAMP WITH TIME ZONE support, the availability of CURRENT_TIMESTAMP could be said to be against the standard—at least confusing. |
MSSQL |
Doesn't have the LOCALTIMESTAMP function.
Instead, it has CURRENT_TIMESTAMP which—however—doesn't return a value of TIMESTAMP WITH TIME ZONE, but rather a value of MSSQL's DATETIME type (which doesn't contain time zone information). |
MySQL | Follows the standard. |
Oracle | Follows the standard. |
Informix | On my TODO. |
Concatenation
Standard |
Core feature ID E021-07: Concatenating two strings is done with the ||
operator:
If at least one operand is NULL, then the result is NULL. It's unclear to me if the DBMS is allowed to try to automatically cast the operands to concatenation-compatible types. |
PostgreSQL |
Follows the standard.
Automatically casts the concatenated values into types compatible with concatenation. If an operand is NULL then the result is NULL. |
DB2 |
Follows the standard, partly.
Does not automatically cast concatenated values into compatible types. |
MSSQL |
Breaks the standard by using the '+' operator instead of '||'.
Does not automatically cast operands to compatible types. If an operand is NULL, then the result is NULL. |
MySQL |
Badly breaks the standard by redefining ||
to mean OR
.
Offers instead a function, Automatically casts values into types which can be concatenated. If an operand is NULL, then the result is NULL. |
Oracle |
Follows the standard, partly.
Automatically casts values into types which can be concatenated. As Oracle interprets NULL as the empty string, it doesn't return NULL if an operand is NULL. |
Informix | Follows the standard.
Automatically casts numeric data into character data, if needed. If an operand is NULL then the result is NULL. |
Constraint handling
The UNIQUE constraint
Standard | As the constraint name indicates,
a (set of) column(s) with a UNIQUE constraint may only contain
unique (combinations of) values.
A column—or a set of columns—which is subject to a UNIQUE constraint must also be subject to a not NULL constraint, unless the DBMS implements an optional "NULLs allowed" feature (Feature ID 591). The optional feature adds some additional characteristics to the UNIQUE constraint:
|
PostgreSQL | Follows the standard, including the optional NULLs allowed feature. |
DB2 | Follows the non-optional parts of the UNIQUE-constraint. Doesn't
implement the optional NULLs allowed
feature.
Documentation (see the unique-constraint section of the page). |
MSSQL | Follows the standard—with a twist:
MSSQL offers the NULLs allowed feature, but allows at most one instance of a NULL-'value', if NULLs are allowed; i.e. breaks characteristic 2 in the above description of the standard. |
MySQL | Follows the standard, including the optional NULLs allowed feature. |
Oracle | Follows the standard—with a twist regarding multiple-column UNIQUE-constraints:
The optional NULLs allowed feature is implemented: If the UNIQUE-constraint is imposed on a single column, then the column may contain any number of NULLs (as expected from characteristic 2 in the above description of the standard). However, if the UNIQUE-constraint is specified for multiple columns, then Oracle sees the constraint as violated if any two rows
|
Informix | On my TODO. |
Mixture of type and operations
Automatic key generation
It's sometimes handy to have the DBMS handle generation of keys . The DBMSes offer various means for this. Note, however, that some database authorities warn against—at least some variants of—auto-generated keys; this is a classic database discourse .
Standard |
The standard specifies a column attribute of: GENERATED ... AS IDENTITY (non-core feature ID T174+T175). When creating a table, an IDENTITY clause may be declared for certain types of columns (INTEGER being one):
or
The column with the IDENTITY attribute will be given values in increasing order, possibly with 'holes' (...,3,4,7,...). A base table may at most contain one column with the IDENTITY attribute. NOT NULL is implied for an IDENTITY column. Normally, a column declared with IDENTITY will also be declared PRIMARY KEY, but it's not implied. The examples differ in their 'ALWAYS' vs. 'BY DEFAULT' clauses:
The standard specifies several extended options which may be declared for a generated IDENTITY column. |
PostgreSQL |
PostgreSQL doesn't support the standard's IDENTITY attribute.
PostgreSQL's best offering for a column with auto-generated values is to declare a column of 'type' SERIAL:
'SERIAL' is a short-hand for creating a sequence and using that sequence to create unique integers for a column. If the table is dropped, PostgreSQL will drop the sequence which was created as a side-effect of using the SERIAL type. As a user may manually
insert or update a value in a column created as SERIAL, this comes
closest to the standard's
If you want semantics like the standard's
Another option is to add the |
DB2 |
Follows standard, albeit with some
restrictions on how identity columns may (not)
be added to an existing table, etc.
Documentation : CREATE TABLE syntax and description of identity columns . |
MSSQL |
MSSQL offers IDENTITY as a column property, but with a different
syntax than the standard's specification.
An example of creating a table with an IDENTITY column:
With MSSQL's IDENTITY attribute, the user cannot manually
insert the value, unless the user has first run
I.e., MSSQL's IDENTITY type is closest to the standard's
Documentation : The IDENTITY property and SET IDENTITY_INSERT . |
MySQL |
MySQL doesn't support the standard's IDENTITY attribute.
As an alternative, an integer column may be assigned the
non-standard
Columns with the AUTO_INCREMENT attribute will—under certain conditions—automatically be assigned a value of <largest value in column>+<at least 1>. Look in MySQL's documentation for the (rather extensive) details. A table can have at most one column with the AUTO_INCREMENT attribute; that column must be indexed (it doesn't have to be a primary key, as in the example SQL above) and cannot have a DEFAULT value attribute. It's probably
not too far fetched to think of MySQL's AUTO_INCREMENT feature as this equivalence: The nice thing about this approach is that the automatic
value insertion should never fail, even though some of the column's
values might have been manually set—i.e. the combined advantages
of the standard's The drawback is that it might result in more house-keeping: The system may need extra table locks when performing row updates/insertions to protect against ghost updates in concurrent transactions—thus slowing down the system in case of many concurrent updates/insertions. |
Oracle |
Oracle doesn't support the standard's IDENTITY attribute.
If you want an auto-incrementing column in Oracle, then create a sequence and use that sequence in a trigger associated to the table. Example: For the table mytable , you want the mytable_id column to be of integer type, with an auto-incrementing values:
This will create an auto-incrementing column resembling the
Note: If 'nice', incrementing values aren't important, you may use Oracle's SYS_GUID function as the default for a column; that way, universally unique identifiers will be assigned if you don't indicate a value for the column in new rows. Documentation
: |
Informix | On my TODO. |
Note: IBM has a page comparing IDENTITY columns and sequences .
Bulk operations
TRUNCATE TABLE
Often, it's useful to be able to remove all rows from a large table in a quick way. And often, DELETE
isn't as quick as you'd like it to be. So several DBMSes implement a TRUNCATE
operation. Typically, truncating means that deletion isn't associated
with triggers which may exist for the table, and typically, truncating
involves little (if any) transaction log activity.
Standard | The SQL standard defines the TRUNCATE TABLE tablename
statement (optional feature ID F200, new in SQL:2008) as:Delete all rows of a base table without causing any triggered action. Unfortunately, the standard doesn't specify
|
PostgreSQL | Follows the standard.
In PostgreSQL, See
the documentation for variations and restrictions. Most importantly,
you need to have be owner of the table to be truncated (or work as a
superuser); alternatively, you need to have TRUNCATE privilege on the
table. Note also the nice—but potentially dangerous— |
DB2 | Almost follows the standard.(since version 9.7)
DB2 requires that the IMMEDIATE
keyword be added the the ordinary TRUNCATE TABLE
statement, e.g.:
TRUNCATE TABLE someschema.sometable
IMMEDIATE
TRUNCATE TABLE
must be the first statement in a transaction. A transaction starting with TRUNCATE TABLE
may include other statements, but if the transaction is rolled back, the TRUNCATE TABLE
operation is not undone.
DB2s TRUNCATE TABLE
operation has a number of optional arguments, see the documentation for more on this; especially, the REUSE STORAGE
argument may be important for ad-hoc DBA tasks.
In DB2 versions < 9.7, you may ab
use the
The Documentation : |
MSSQL | Follows the standard.
In MSSQL, You need to have at least ALTER-permission on the table to be truncated. |
MySQL | MySQL has a TRUNCATE TABLE
statement, but it doesn't always follow the standard.
Note that in some cases, MySQL's truncate command is really the equivalent of an unrestricted DELETE command (i.e.: potentially slow and trigger-invoking). Its behaviour depends on which storage engine the table is managed by. When using InnoDB (transaction safe) tables, |
Oracle | Follows the standard.
Note, that the You may find that Needed privileges—Quoting from the documentation:
|
Informix | On my TODO. |
Command line procedures / metadata
The following are not necessarily SQL operations, but rather a description of how different operations are performed in the command line interface provided by each product.
The shape of the command line interfaces in the commercial products is depressing. Vendors, please do something about it: Not all database developers like to use slow GUIs for technical stuff. And sometimes, DBMS work is performed over slow Internet lines which makes a decent command line interface vital.
Fortunately, a tool like HenPlus exists. It can be a pain to install, but once working, it's nice to work with.
Starting the command line interface
Standard | Not defined. |
PostgreSQL |
Run:psql
which should be in the PATH in any sensible installation. PostgreSQL's command line interface is very user friendly. It has command history (press arrow-up for previous commands) and a fairly well-working command completion feature. |
DB2 |
Run:db2 -t
(The -t
argument tells the command line processor to a semicolon as statement
terminator instead of the default (newline). This allows for multi-line
SQL statements.)
The The 'utility' doesn't seem to have anything resembling useful
command history or command completion. Fortunately, queries may be
sent to the DB2 also has a 'utility' called |
MSSQL |
The command line interface is started by runningsqlcmd
In MSSQL 2000, the command line interface was started by running An alternative to osql—apart from HenPlus, mentioned above—is SQSH which should work on any modern open source operating system, except it doesn't seem to support Kerberos, so you need to log into the database using a database-account (not a Windows-account). |
MySQL |
Run:mysql
If you need help on the optional command line options, see the man page. On platforms like Linux and FreeBSD (which have decent readline-capabilities), MySQL's command line interface is simply great; not much else to say. MySQL's command line interface is said to be rather poor on Windows, though. |
Oracle |
Run:sqlplus
A
unique feature of Oracle is that a web-based administration interface
is provided, as a supplement to the local administration software. The
URL to the interface is typically |
Informix | Informix' command line utility is called dbaccess
.
If run without arguments, it starts a menu system. If you simply want
to shoot SQL statements off to the database, another form is more
convenient, at least on unix: echo 'SELECT foo FROM bar' | dbaccess databasename
|
Getting a list of databases
Standard | Not specified, as far as I know. (By the way: The SQL standard doesn't have the concept of a database as a container of schemas; instead, the standard specifies that schemas are contained in a catalog .) |
PostgreSQL | Using SQL: SELECT datname FROM pg_catalog.pg_database
When working in the Alternative (when working from the terminal, not in Documentation : The psql tool, the pg_database catalog. |
DB2 | Offers the LIST DATABASE DIRECTORY
command, but only when working in the db2
command line processor (i.e. not when working from db2batch
); this command's output is human readable, but sub-optimal as machine readable format.
|
MSSQL | EXEC SP_HELPDB
|
MySQL | SHOW DATABASES
|
Oracle | In Oracle, there is a one-to-one relationship between databases
and instances
(unless you work with a clustered Oracle system). You can get a list of
instances; the way to do it depends on the operating system which
Oracle is running on:
Documentation: oratab |
Informix | Connect to the sysmaster
database (all users are allowed to do this) and run:
SELECT name FROM sysmaster:sysdatabases
|
Getting a list of schemas
Standard | SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA
|
PostgreSQL | In the command line interface: /dn
or /dn+
(for more details).
Using SQL: Follows the standard. Documentation : |
DB2 |
SELECT schemaname FROM syscat.schemata
|
MSSQL | Follows the standard. |
MySQL | MySQL doesn't support schemas. |
Oracle | Oracle
has a peculiar approach to schemas: A schema exists for each and every
user. And there cannot be a schema without a corresponding user.
Consequently, a way to get a list of schemas in Oracle is to query the ALL_USERS
dictionary view:
SELECT username FROM all_users
|
Informix | Informix' concept of schemas is closely related to user names, so—somewhat surprisingly—the query is:
SELECT UNIQUE owner FROM systables WHERE tabid>99
|
Getting a list of tables
Standard |
Part 11 of the SQL standard specifies the INFORMATION_SCHEMA schema which must
be part of all database catalogues. The schema may be used like this:
or (often more relevant):
See a warning about potential case sensitivity problems below. |
PostgreSQL |
Follows the standard, except for
some gotchas
mentioned below.
In command-line context, it's easier to use the following non-SQL command
instead of querying the INFORMATION_SCHEMA: Documentation: The |
DB2 |
Doesn't provide the standard INFORMATION_SCHEMA
. Instead, DB2 offers
the SYSCAT
schema (catalog) which is somewhat compatible.
Offers what is probably a shorthand to some system catalog query: |
MSSQL | Follows that standard. Sometimes, the SP_TABLES
system stored procedure is
easier to use.
Documentation :
|
MySQL |
Follows the standard, except that MySQL doesn't support schemas, so one might say that MySQL's INFORMATION_SCHEMA
is really an 'INFORMATION_DATABASE
' or 'INFORMATION_CATALOGUE
'.
In command-line context, it's easier to use the following non-standard SQL:
Documentation : |
Oracle |
Doesn't provide the standard INFORMATION_SCHEMA. Provides a data dictionary
system instead.
The quickest way to get a usable list of 'normal' tables in the current schema: |
Informix | Doesn't provide the standard INFORMATION_SCHEMA
out of the box. A few of the standard's INFORMATION_SCHEMA views may be added by running a special script, though.
Informix offers a set of system catalogs
instead. To get a list of tables:
The above query will include views and other objects; if you want base tables only:
|
Warning about a general case sensitivity gotcha
Note that there may be case sensitivity issues involved when using meta-data
views like those in the INFORMATION_SCHEMA. Generally,
the standard states that the name of an identifier (such as table names) are
implicitly converted to uppercase, unless double-quotes are used when referring
to the identifier. The same goes for identifiers used in queries: A query like
SELECT foo FROM tablename
is implicitly converted to
SELECT FOO FROM TABLENAME
.
If you create your table as
CREATE TABLE testtab (id INTEGER PRIMARY KEY)
then a query like
SELECT * FROM testtab
should work fine, and
SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME='TESTTAB'
should work, while the following query will probably fail:
SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME='testtab'
Warning about INFORMATION_SCHEMA gotchas in PostgreSQL
Warning: PostgreSQL's case-conversion rules for unquoted identifiers (such as table names) are non-standard: PostgreSQL converts the identifiers to lower case , instead of converting to upper case . This means that you may try altering the case of identifier names used for queries in the INFORMATION_SCHEMA if you experience unexpected, empty metadata queries.
Note also that due to PostgreSQL's handling of constraint names, the INFORMATION_SCHEMA cannot safely be used to deduce referential constraints ; for this, you have to use PostgreSQL's pg_catalog system-schema.
Getting a table description
Standard |
Part 11 of the SQL standard specifies the INFORMATION_SCHEMA schema which must
be part of all database catalogues. The schema may be used like this:
—or like this (more verbose):
To get information about constraints, involved columns and (possibly)
referenced columns, a query like this may be used: If you don't care about potential namespace conflicts, you may leave out the lines commented with " See also: Warning about potential case sensitivity problems above. |
PostgreSQL |
Follows the standard, except for
some gotchas
mentioned above.
In command-line context it's easier to use this non-SQL command: |
DB2 |
Doesn't provide the standard INFORMATION_SCHEMA.
To obtain (very) basic information about a table: To get information about constraints, including involved/referred columns,
a query like the following may be used, although the Documentation :
|
MSSQL | Follows the standard, except that
Often, the Documentation : |
MySQL |
Follows the standard, except that
In command-line context it's easier to use this non-SQL command: Documentation : |
Oracle |
Doesn't provide the standard INFORMATION_SCHEMA. Offers data dictionary views
instead.
To get (very) basic information: To get information on constraints, including foreign (referred) table/column
information, a query like this may be used (adjust tablename
in one of the last lines): To
get information
on indexes on a table, a query like this may be
used (adjust tablename
in one of the last lines): Documentation :
|
Informix | Doesn't provide the standard INFORMATION_SCHEMA
out of the box. If a special script is run, an INFORMATION_SCHEMA may be added which allows for using the most basic
standards-based table description query.
In practice, an Informix-only query is used. The following query provides very basic table information, excluding constraints:
Notice that the table name is in lower case. The colname values are numeric codes which need to be looked up in order to provide meaning. |
Manually telling the DBMS to collect statistics
In most DBMSes, it's possible to enable automatic statistics gathering, but sometimes, it's nice to be able to manually tell the DBMS to gather statistics for a table (or a number of tables).
Standard | Not standardized. |
PostgreSQL | ANALYZE tablename
If the tablename parameter is left out, then statistics are gathered for all tables in the current database. |
DB2 | RUNSTATS ON TABLE schema-name
.table-name
AND INDEXES ALL
(many variations/options available) The Documentation
: |
MSSQL | First, you have to add statistics to the table:
CREATE STATISTICS stats_name
(The CREATE STATISTICS
step is not needed for indexed columns. Thus, this step may be skipped
if you are satisfied with keeping statistics on indexed columns only.)
The statistics may then be updated when needed:
Having to explicitly mention tables and columns can be tedious, and in many cases, the Documentation : CREATE STATISTICS , UPDATE STATISTICS , sp_createstats , sp_updatestats |
MySQL | ANALYZE TABLE tablename
|
Oracle | Oracle offers to estimate
(quick) or compute
(thorough)
statistics for a database object. The quick way to do this is to use the deprecated
ANALYZE
command which can be used in various ways, e.g.
—Or: If you want to stay away from deprecated features (although I doubt that Oracle will remove ANALYZE...STATISTICS... any time soon), you need to use the DBMS_STATS package . |
Informix | On my TODO. |
Getting a query explanation
Standard | Not standardized. |
PostgreSQL | EXPLAIN <query>
|
DB2 | The
easiest way to get a query explanation is to save the query in a file
(without a terminating semicolon), and then run a special command-line
utility:db2expln -database databasename
-stmtfile query.sql
-terminator ';' -terminal
In the above example, the query has been saved to a file called "query.sql". In some situations, you may want to use the If you prefer to get the explanation through SQL:
|
MSSQL | MSSQL can be put in a query explanation mode where queries
are not actually executed, but a query explanation is returned instead:SET SHOWPLAN_TEXT ON
The query explanation mode is turned off by running |
MySQL | EXPLAIN <query>
|
Oracle | EXPLAIN PLAN FOR <query>
After the query has run, do the following to get the plan explanation: SELECT plan_table_output FROM table(dbms_xplan.display())
|
Informix | On my TODO. |
Turning on query timing
Standard | Not standardized. |
PostgreSQL | /timing
|
DB2 | Run the query in the "db2batch
" command line processor; db2batch
prints the elapsed time of each query.
|
MSSQL | SET STATISTICS TIME ON
|
MySQL | MySQL's command line interface prints query times by default. |
Oracle | SET TIMING ON
|
Informix | On my TODO. |
JDBC
JDBC driver jar file name, and general documentation
PostgreSQL | The PostgreSQL JDBC Driver : postgresql-postgresqlversion -jdbcbuild# .jdbc4.jar |
DB2 | IBM Data Server Driver for JDBC : db2jcc.jar (included in default DB2 client software installations; may also be downloaded separately, after registration) |
MSSQL | Microsoft's driver
: sqljdbc.jar
Alternative: The open source JTDS driver : jtds-version .jar Documentation : |
MySQL | The MySQL Connector/J driver: mysql-connector-java-version -bin.jar |
Oracle | Oracle's JDBC drivers : ojdbc5.jar (for Java 5), ojdbc6.jar (for Java 6) |
Informix | IBM's Informix JDBC driver
:
ifxjdbc.jar (download requires registration and filling out annoying
questionnaires, and an installer which only works with some JREs has to
be run to unpack the driver) |
JDBC driver class name
PostgreSQL | org.postgresql.Driver |
DB2 | com.ibm.db2.jcc.DB2Driver |
MSSQL | Microsoft's driver: com.microsoft.sqlserver.jdbc.SQLServerDriver |
MySQL | com.mysql.jdbc.Driver |
Oracle | oracle.jdbc.driver.OracleDriver |
Informix | com.informix.jdbc.IfxDriver |
JDBC connection URL
PostgreSQL | jdbc:postgresql://hostname /DBname |
DB2 | jdbc:db2://hostname
:50000
/DBname
or (if the database is on the local host): jdbc:db2:DBname |
MSSQL | On my TODO. |
MySQL | jdbc:mysql://[host ][,failoverhost ][:port ]/[database ]?user=username &password=password |
Oracle | jdbc:oracle:thin:@hostname :1521 :instancename |
Informix | jdbc:informix-sqli://hostname
:9088
/DBname
:INFORMIXSERVER=instancename
Use port 1526 instead of 9088 if the Informix version is <11. |
Other topics
Dummy table use
Some DBMSes let you perform a query like this:
SELECT 1+1
answering
2
With other DBMSes, you need to insert a dummy-table expression to obtain the same result:
SELECT 1+1 FROM dummy-table
Standard | On my TODO. |
PostgreSQL | No need for dummy-table.
In addition, the |
DB2 | Dummy-table: SYSIBM.SYSDUMMY1
.
In addition, the |
MSSQL | No need for dummy-table. |
MySQL | No need for dummy-table, although MySQL allows you to
refer to a DUAL
dummy-table (for Oracle compatibility). |
Oracle | Dummy-table: DUAL
. |
Informix | Informix requires that you include a FROM
specification. In recent versions of Informix(since version 11.10)
, a dummy table has been included: sysmaster:sysdual
.
For older Informix versions, the tradition is to use code like:
Documentation : |
Obtaining DBMS version
Standard | SELECT CHARACTER_VALUE
|
PostgreSQL | Follows the standard. An alternative, non-standard function may be used:
SELECT VERSION()
|
DB2 | SELECT service_level FROM SYSIBMADM.ENV_INST_INFO
—or run the special Documentation
: |
MSSQL | MSSQL's
implementation of the IMPLEMENTATION_SCHEMA doesn't seem to include the
SQL_IMPLEMENTATION_INFO view. In stead, you may useSELECT SERVERPROPERTY('ProductVersion')
(just the version), or SELECT @@VERSION
(verbose, harder to parse). Documentation : SERVERPROPERTY , @@VERSION |
MySQL | MySQL's INFORMATION_SCHEMA
doesn't include the SQL_IMPLEMENTATION_INFO
view.
Work-around: |
Oracle | SELECT banner FROM v$version
|
Informix | Using SQL: SELECT dbinfo('version','full') FROM systables WHERE tabid=1
From the command line:
Documentation :
|
Standard TCP/IP port
Product | Port# | Notes | Documentation |
---|---|---|---|
Standard | Not specified | ||
PostgreSQL | 5432 | For security reasons, PostgreSQL doesn't listen to non-local TCP interfaces by default. | Documentation |
DB2 | 50000 | ||
MSSQL | 1433 | By default, MSSQL Express Edition doesn't listen for TCP connections. | |
MySQL | 3306 | ||
Oracle | 1521 | Documentation | |
Informix | 9088 (unencrypted) | Informix versions prior to version 11, the default port was 1526. |
Diagnostic log
Each DBMS has different ways to record diagnostic information (event logs).
Standard | Not covered by the standard. |
PostgreSQL | By
default, PostgreSQL logs to stderr, meaning that it's highly
installation specific where the dianostic information is put; on this
author's system, the default ends up in /var/lib/pgsql/pgstartup.log
. The default can be set to something more reasonable (such as syslog
on unix, eventlog
on Windows) by adjusting the log_destination
configuration parameter.
|
DB2 | On unix systems, DB2s diagnostic log file is called db2diag.log
and lives in the sqllib/db2dump
sub-directory of the instance user's home directory. I.e., a typical full path is:/home/db2inst1/sqllib/db2dump/db2diag.log
If the file is renamed or deleted, DB2 will create a new db2diag.log
without having to be restarted.
|
MSSQL | On my TODO. |
MySQL | On my TODO. |
Oracle | A diagnostic directory contains a file called alert_INSTANCE
.log
. The diagnostic directory is determined by the following query:SELECT value FROM v$parameter WHERE name='background_dump_dest'
Examples of the diagnostic directory:
|
Informix | The
path of the diagnostic log is defined by the MSGPATH configuration
parameter. On a Linux installation, using default options:/opt/IBM/informix/tmp/online.log
|
Related work
- Mimer Information Technology AB (makers of the
Mimer SQL
DBMS) has an
interesting
feature comparison chart
, displaying what SQL:1999 features are implemented
in different commercial products. May be biased because it's created by a DBMS vendor.
Mimer also has lists of reserved words . - Wikipedia has a Comparison of relational database management systems page. And a Wikibook called SQL dialects reference is in the works.
- Chris Fehily's SQL: Visual QuickStart Guide teaches SQL by first describing the standards-based (SQL:2003) approach, and then how to adjust to the real World, using MS Access, MSSQL, Oracle, MySQL, PostgreSQL, and DB2. (Full disclosure note: I was technical editor on second edition of the book.)
- Alessandro Tanasi: Database datatype comparison sheet .
- Peter Gulutzan (who works for MySQL AB) has written several
articles
related to the subject. He has also written two related books:
- SQL-99 Complete, Really (co-authored with Trudy Pelzer) is said to be good.
- SQL Performance Tuning (also co-authored with Trudy Pelzer), mentions quite a few cross-product SQL issues (primarily related to performance, of course).
- Some
DBMS evaluations performed at the Astrogrid Virtual Observatory (focus on spatial functionality):
- Various comparisons of MySQL, PostgreSQL and DB2 .
- Comparison of availability and names of mathematical functions in major DBMS products.
- Autumn '03: Comparison of DB2, MySQL, and Postgres , comparing ease of use, scalability and performance of two types of spatial joins.
- Autumn '02: Comparison of PostgreSQL, MySQL, Oracle, SQL Server and DB2 .
- Oracle / SQL Server / DB2 / Mckoi / MySQL Database Equivalents .
- Uday Parmar: Open Source Database Feature Comparison Matrix
.
Note: Created by employees of a database vendor. - SQLite's survey of NULL-handling: NULL Handling in SQLite Versus Other Database Engines .
- Bowman/Emerson/Darnovsky's The Practical SQL Handbook—Using SQL Variants is OK for this subject, although it is rather out-dated and (worse) doesn't include any guidance on working with open source DBMSs.
- Kevin E. Kline's SQL in a Nutshell from O'Reilly is a good reference. O'Reilly has also published Jonathan Gennick's SQL Pocket Guide which looks good (but I haven't read it).
- The Analysis and Solutions Company:
- Building Truly Portable Database Applications in PHP includes advice on DBMS differences (some of the presentation is PHP-specific, as the title indicates).
- Database Portability: Date and Timestamp Columns .
- Jutta Horstmann OSDBmigration .
- Lewis Cunningham: A comparison of data types between various databases - Oracle, MySQL, DB2, Ingres, SQL Server, Firebird, Postgres .
- Bristle Software SQL Tips contains tips with variants for several DBMSes.
- ConnStr.net: .Net/ODBC connection strings to all sorts of database systems .
- See also my DBMS links .
Acknowledgments
The following people have provided comments, suggestions and/or fixes, resulting in content changes on this page:
- Ian Barwick
- Chester Kustarz
- Bruno Wolff III
- Carsten Pedersen
- Jürgen Auer
- Edi Stocker
- Tzvetan Tzankov
- Jess Robinson
- Gordon P. Hemsley
- Philip Nelson
- Andreas Plesner Jacobsen
- Clive Page
- Holger Jakobs
- Dennis Björklund
- Chris Fehily
- Alf-Ivar Holm
- Joseph Fuda
- J M Sykes
- Greg Sabino Mullane
- Jari Aalto
- Robert Jones
- Greg Fortune
- Dick Leone
- Neil Conway
- Markus Schaber
- James Denny
- Neal Lindsay
- David Rowley
- Erik Berglund
- Michael Swart
- Rados艂aw Zieli艅ski
- Jonathan Giroux
- Thomas Kellerer
- Guillaume Taglang
- Claus Samuelsen
- David Fetter
- Mike Wilson
- James Beckett
- Joachim Selke
(In chronological order.)
To Troels' home page .