去重
比如说有一个表，字段id（自增主键），book_id，line_num记录某本书看到某一行，
id book_id line_num
1 123 110
2 123 110
3 222 111
4 311 322
5 311 322
6 311 322
去除重复数据，sql如下：
""" DELETE
FROM
test3
WHERE
id NOT IN (
SELECT
a.id
FROM
(
SELECT
min(id) AS id
FROM
test3
GROUP BY
book_id,
line_num
HAVING
count(*) > 1
) a
)
"""
两个子查询是为了避免You can't specify target table 'test3' for update in FROM clause 错误
DDL DML
Data Definition Language (DDL) statements are used to define the database structure or schema. Some examples:
CREATE - to create objects in the database
ALTER - alters the structure of the database
DROP - delete objects from the database
TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed
COMMENT - add comments to the data dictionary
RENAME - rename an object
Data Manipulation Language (DML) statements are used for managing data within schema objects. Some examples:
SELECT - retrieve data from the a database
INSERT - insert data into a table
UPDATE - updates existing data within a table
DELETE - deletes all records from a table, the space for the records remain
MERGE - UPSERT operation (insert or update)
CALL - call a PL/SQL or Java subprogram
EXPLAIN PLAN - explain access path to data
LOCK TABLE - control concurrency
http://www.orafaq.com/faq/what_are_the_difference_between_ddl_dml_and_dcl_commands
