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
原文：http://www.cnblogs.com/ruiy/p/SQL.html