What is SQL

SQL was, is and always will be the database language for relational database systems such as Oracle, DB2, Sybase, Informix and Microsoft SQL Server. The first questions to ask are what is SQL and how do you use it with databases? SQL has four main roles:

  • Creating a database and defining its structure (DDL)
  • Querying/Modifying the existing data in the database (DML)
  • Controlling database security (DCL)
  • Transaction control (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

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

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