Oracle 语言分类（DDL，DML，DCL，TCL）

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Oracle的语言分四类，它们是（数据定义语言DDL）、（数据操纵语言DML）、（数据控制语言DCL） 和 (事务控制语言TCL)
DDL　　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
DML
　　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
DCL
　　Data Control Language (DCL) statements. Some examples:
　　GRANT - gives user's access privileges to database
　　REVOKE - withdraw access privileges given with the GRANT command
TCL
　　Transaction Control (TCL) statements are used to manage the changes made by DML statements. It allows statements to be grouped together into logical transactions.
　　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 isolation level and what rollback segment to use