DML—-Data Manipulation Language 数据操纵语言
DDL—-Data Definition Language 数据库定义语言
如 create procedure之类
DCL—-Data Control Language 数据库控制语言
  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
  Data Control Language (DCL) statements. Some examples:
  GRANT – gives user’s access privileges to database
  REVOKE – withdraw access privileges given with the GRANT command
  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


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