Introduction to SQL
Structure Query Language(SQL) is a database query language used for storing and managing data in Relational DBMS. SQL was the first commercial language introduced for E.F Codd's Relational model of database. Today almost all RDBMS(MySql, Oracle, Infomix, Sybase, MS Access) use SQL as the standard database query language. SQL is used to perform all types of data operations in RDBMS.
SQL defines following ways to manipulate data stored in an RDBMS.
DDL: Data Definition Language
This includes changes to the structure of the table like creation of table, altering table, deleting a table etc.
All DDL commands are auto-committed. That means it saves all the changes permanently in the database.
|create||to create new table or database|
|truncate||delete data from table|
|drop||to drop a table|
|rename||to rename a table|
DML: Data Manipulation Language
DML commands are used for manipulating the data stored in the table and not the table itself.
DML commands are not auto-committed. It means changes are not permanent to database, they can be rolled back.
|insert||to insert a new row|
|update||to update existing row|
|delete||to delete a row|
|merge||merging two rows or two tables|
TCL: Transaction Control Language
These commands are to keep a check on other commands and their affect on the database. These commands can annul changes made by other commands by rolling the data back to its original state. It can also make any temporary change permanent.
|commit||to permanently save|
|rollback||to undo change|
|savepoint||to save temporarily|
DCL: Data Control Language
Data control language are the commands to grant and take back authority from any database user.
|grant||grant permission of right|
|revoke||take back permission.|
DQL: Data Query Language
Data query language is used to fetch data from tables based on conditions that we can easily apply.
|select||retrieve records from one or more table|