关系数据库的语言分类
时间：2010-09-13 来源：叶振华
关系数据库的语言分三类，它们是（数据描述语言DDL）、（数据操纵语言DML）和（数据控制语言DCL），而SQL是（集DDL、DML、DCL为一体）的标准关系数据库语言。
DDL is Data Definition Language statements. Some examples:
DDL is Data Definition Language statements. 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
- GRANT - gives user's access privileges to database
- REVOKE - withdraw access privileges given with the GRANT command
- 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
- CALL - call a PL/SQL or Java subprogram
- EXPLAIN PLAN - explain access path to data
- LOCK TABLE - control concurrency
- COMMIT - save work done
- SAVEPOINT - identify a point in a transaction to which you can later roll back
- ROLLBACK - restore database to original since the last COMMIT
- SET TRANSACTION - Change transaction options like what rollback segment to use
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