MySQL2023-03-25

I already have a student table, how can I create a table with same structure?

To create a new table with the same structure as an existing table in MySQL, you can use the CREATE TABLE statement with the LIKE clause, like this:

sql

CREATE TABLE new_table LIKE old_table;

This will create a new table called new_table with the same column names, data types, and constraints as the existing table called old_table. However, it will not copy any data from the old table.

If you want to copy both the structure and the data from the old table to the new table, you can use the CREATE TABLE statement with the AS SELECT clause, like this:

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CREATE TABLE new_table AS SELECT * FROM old_table;

This will create a new table called new_table with the same column names, data types, and constraints as the existing table called old_table, and it will also copy all the data from the old table to the new table.

 

 How to create temp table in mysql?

In MySQL, you can create a temporary table by using the CREATE TEMPORARY TABLE statement. Here's the basic syntax:

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CREATE TEMPORARY TABLE temp_table_name ( column1 datatype1, column2 datatype2, ... );

You can specify the column names and data types for the temporary table just like you would for a regular table. The only difference is the TEMPORARY keyword, which tells MySQL to create a temporary table that only exists for the duration of the current session.

Here's an example of creating a simple temporary table:

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CREATE TEMPORARY TABLE temp_sales ( id INT, date DATE, amount DECIMAL(10,2) );

This will create a temporary table called temp_sales with three columns: id, date, and amount. The data types for these columns are INT, DATE, and DECIMAL(10,2), respectively.

 how to use desc command in mysql?

 

In MySQL, the DESC command is used to describe the structure of a table. It shows information about the columns in the table, such as the column name, data type, whether it allows null values, and any indexes that are defined on the column.

The basic syntax for using the DESC command is as follows:

sql

DESC table_name;

Here's an example of how to use the DESC command:

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DESC employees;

This will show the structure of the employees table, including the name, data type, nullability, and any indexes for each column in the table.

You can also use the DESCRIBE command as a synonym for DESC. So, the following two commands are equivalent:

sql

DESC table_name;

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DESCRIBE table_name;

Note that the output of the DESC command is not as detailed as the output of the SHOW CREATE TABLE command, which shows the full table definition including the table's create statement.

how to use show create table in mysql?

In MySQL, the SHOW CREATE TABLE command is used to display the complete CREATE TABLE statement that was used to create a table. This includes all the column definitions, indexes, and other options that were specified when the table was created.

Here's the basic syntax for using the SHOW CREATE TABLE command:

sql

SHOW CREATE TABLE table_name;

Here's an example of how to use the SHOW CREATE TABLE command:

sql

SHOW CREATE TABLE employees;

This will display the complete CREATE TABLE statement that was used to create the employees table. The output will include all the column definitions, indexes, and other options that were specified when the table was created.

You can also use the output of the SHOW CREATE TABLE command to create a new table with the same structure as the original table. To do this, simply copy the output of the command and use it as the CREATE TABLE statement for the new table.

alter table syntax in mysql?

In MySQL, the ALTER TABLE command is used to modify the structure of an existing table. Here's the basic syntax for using the ALTER TABLE command:

ALTER TABLE table_name
ADD column_name data_type [NOT NULL] [DEFAULT default_value] [FIRST | AFTER column_name],
MODIFY column_name data_type [NOT NULL] [DEFAULT default_value] [FIRST | AFTER column_name],
DROP column_name,
ADD [UNIQUE | FULLTEXT | SPATIAL] INDEX index_name (column_list),
DROP INDEX index_name,
CHANGE old_column_name new_column_name data_type [NOT NULL] [DEFAULT default_value] [FIRST | AFTER column_name],
RENAME new_table_name;

Let's go through each of these clauses in detail:

• ADD: Used to add a new column to the table. You must specify the column name and data type. You can also specify whether the column is NOT NULL and a default value for the column.

• MODIFY: Used to change the data type of an existing column. You must specify the column name and the new data type. You can also specify whether the column is NOT NULL and a default value for the column.

• DROP: Used to remove a column from the table. You only need to specify the column name.

• ADD INDEX: Used to add a new index to the table. You can specify the index type (UNIQUE, FULLTEXT, or SPATIAL), the index name, and the column or columns to be indexed.

• DROP INDEX: Used to remove an index from the table. You only need to specify the index name.

• CHANGE: Used to rename an existing column. You must specify the old column name, the new column name, and the new data type. You can also specify whether the column is NOT NULL and a default value for the column.

• RENAME: Used to rename the table. You must specify the new table name.

You can use one or more of these clauses in a single ALTER TABLE statement. For example, to add a new column and rename an existing column in the same ALTER TABLE statement, you could use the following syntax:

ALTER TABLE table_name
ADD new_column_name data_type,
CHANGE old_column_name new_column_name data_type;

Note that the order of the clauses in the ALTER TABLE statement is important. For example, if you try to add a new column after a non-existent column or specify an invalid data type, the statement will fail.

how to use insert, update, delete and truncate in mysql

Here are some examples of how to use the INSERT, UPDATE, DELETE, and TRUNCATE statements in MySQL:

1. INSERT: To insert data into a MySQL table, you can use the INSERT INTO statement followed by the table name and the values to be inserted. Here's an example:

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INSERT INTO my_table (column1, column2, column3) VALUES ('value1', 'value2', 'value3');

2. UPDATE: To update data in a MySQL table, you can use the UPDATE statement followed by the table name, the SET keyword, and the new values to be updated. Here's an example:

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UPDATE my_table SET column1='new_value1', column2='new_value2' WHERE some_condition;

3. DELETE: To delete data from a MySQL table, you can use the DELETE FROM statement followed by the table name and the condition to determine which rows to delete. Here's an example:

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DELETE FROM my_table WHERE some_condition;

4. TRUNCATE: To remove all data from a MySQL table, you can use the TRUNCATE TABLE statement followed by the table name. Here's an example:

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TRUNCATE TABLE my_table;

It's important to note that the TRUNCATE statement is different from the DELETE statement in that it removes all data from the table, while DELETE only removes specific rows based on a condition. Additionally, the TRUNCATE statement is generally faster and uses fewer system resources than the DELETE statement.

 

character set vs charset

"Character set" and "charset" are two terms used interchangeably in the context of databases and programming languages to refer to a set of characters and their corresponding encodings. Both terms refer to the same concept, and there is no functional difference between them.

In the MySQL documentation, both terms are used to refer to character sets, and the terms are used interchangeably. Similarly, in programming languages such as PHP and Python, both terms are used to refer to character sets.

Ultimately, whether you use "character set" or "charset" comes down to personal preference or the convention used in your particular context.

When to use the comment of the columns?

Using comments for columns in a database can be helpful in several scenarios:

1. Documenting the purpose of a column: By adding a comment to a column, you can describe the purpose of the column and its expected values. This can help other developers or DBAs who are unfamiliar with the database to understand the data structure and how to work with it.

2. Providing additional information: In some cases, a column may require additional information that cannot be captured through its name or data type. For example, a column might represent a foreign key relationship to another table or an external data source. By including a comment, you can provide more information about the column's usage and data source.

3. Maintaining the database: Comments can also be useful for maintaining the database. For example, if a column is no longer needed or is going to be deprecated, you can add a comment to document this change. This can help prevent other developers from accidentally using the column in new code and can make it easier to update the database schema over time.

Overall, using comments for columns can help to improve the clarity and maintainability of a database over time.

INT UNSIGNED vs int(10)

INT UNSIGNED and INT(10) are both data types in MySQL, but they serve different purposes.

INT UNSIGNED is a type of integer data that can store non-negative numbers (i.e., zero and positive integers) in the range of 0 to 4,294,967,295. The UNSIGNED keyword specifies that the integer should not have a sign (i.e., it should always be positive).

INT(10) specifies the display width of the integer. It does not change the range of values that can be stored in the column. The number in parentheses is the number of digits that should be displayed when the value is retrieved. It is mainly used for display purposes and does not affect the storage size or range of the data type.

In summary, INT UNSIGNED is used to store non-negative integers while INT(10) is used to specify the display width of the integer.

 

column type changed from varchar(30) to varchar(100), should I use modify or change?

In MySQL, the ALTER TABLE statement can be used to modify the structure of an existing table. When changing the data type of a column, you can use either MODIFY or CHANGE.

If you want to change the data type of a column while keeping the column name the same, you can use the MODIFY clause, like this:

sql

ALTER TABLE mytable MODIFY mycolumn VARCHAR(100);

If you want to change both the data type and the column name, you can use the CHANGE clause, like this:

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ALTER TABLE mytable CHANGE oldcolumn newcolumn VARCHAR(100);

So, if you only want to change the length of the varchar column from 30 to 100, you can use the MODIFY clause.

 can I set oldcolumn and newcolumn with same name to modify type

No, you cannot set the old column and new column with the same name when modifying the column type in MySQL. When modifying the column type, you need to use the MODIFY keyword, followed by the column name and the new data type.

For example, to modify a column named my_column from VARCHAR(30) to VARCHAR(100), you can use the following command:

sql

ALTER TABLE my_table MODIFY my_column VARCHAR(100);

You cannot use the CHANGE keyword in this scenario since it is used to rename a column while also changing its data type or other attributes.

 

delete table where vs delete from table where

In MySQL, both DELETE table WHERE and DELETE FROM table WHERE can be used to delete data from a table based on a specified condition. There is no functional difference between the two, they are just different syntaxes for achieving the same result.

Here are some examples of both syntaxes

-- Using DELETE table WHERE
DELETE mytable WHERE id = 5;

-- Using DELETE FROM table WHERE
DELETE FROM mytable WHERE id = 5;

Both of the above examples will delete the row(s) where the id column equals 5 in the mytable table.

when to use int and int unsigned

In MySQL, INT is a data type that stores whole numbers (integers) and takes up 4 bytes of storage. The range of values that can be stored in INT is from -2,147,483,648 to 2,147,483,647.

INT UNSIGNED is a variation of the INT data type that only stores non-negative (unsigned) integers, i.e., it can store values from 0 to 4,294,967,295. By using INT UNSIGNED, you can effectively double the range of positive values that can be stored in the same 4 bytes of storage.

You should use INT UNSIGNED when you know that the values to be stored will never be negative. This is typically the case when you are storing IDs, counts, or quantities that are always positive. However, if there is a possibility of storing negative values, you should use INT.

It is important to choose the appropriate data type for your columns in order to avoid wasting storage space or running into data overflow issues.

 

How many decimal places does the max number of int unsigned have?

The maximum value of INT UNSIGNED in MySQL is 4294967295, which requires 10 digits to represent. Since INT UNSIGNED does not allow negative values, all 10 digits are used to represent the magnitude of the number.

INT UNSIGNED does not include decimal places as it represents a whole number between 0 and 4294967295. If you need to store a number with decimal places, you should use the DECIMAL data type instead.

 

when to use int(3) as int unsigned  occupy 10 decimal palaces?

Using int(3) as int unsigned will still occupy 10 decimal places because the int data type in MySQL is always 4 bytes (32 bits) regardless of the display width specified.

The display width specified with int(3) only affects the zero-padding of the display of the integer value, but it does not affect the storage size or range of values that can be stored in the column.

Therefore, int(3) as int unsigned is not useful in terms of storage or range of values. It might be used for display purposes, but it can be misleading if the actual values stored in the column exceed 3 digits.

 

 if I have int number 1254 and set int(3), how will this number be displayed?

If you set an int field to int(3), it will not affect the range of values that can be stored in the field, but it will affect the display width when the value is displayed.

In the case of the number 1254, it would be stored in the field without any issues since it's within the range of values that can be stored in an int field. However, when you retrieve this value, it will be displayed as 1254, with a display width of 4, even though you specified the column as int(3). This is because the number itself takes up 4 digits, and the display width is only a hint for display purposes, not a constraint on the range of values that can be stored.

 

Does it mean that the number 1 will be displayed as 001 when type in int(3)

Yes, that's correct. When you define an integer column as int(3) and insert a value of 1, it will be displayed as 001. However, please note that the length parameter in int(3) only affects the display width, not the actual storage size or range of the integer value.

 

使用这个语句修改字段类型

alter table seller

modify id int unsigned;

 

 

 

 

 

 

 

 

作者:Chuck Lu    GitHub