What is SQL?

Structured Query Language

SQL stands for Structured Query Language. SQL language is used to create, transform and retrieve information from RDBMS (Relational Database Management Systems). SQL is pronounced SEQUEL. SQL was developed during the early 70’s at IBM.

Most Relational Database Management Systems like MS SQL Server, Microsoft Access, Oracle, MySQL, DB2, Sybase, PostgreSQL and Informix use SQL as a database querying language.

Even though SQL is defined by both ISO and ANSI there are many SQL implementation, which do not fully comply with those definitions. Some of these SQL implementations are proprietary. Examples of these SQL dialects are MS SQL Server specific version of the SQL called T-SQL and Oracle version of SQL called PL/SQL.

SQL is a declarative programming language designed for creating and querying relational database management systems. SQL is relatively simple language, but it’s also very powerful.

SQL can insert data into database tables. SQL can modify data in existing database tables. SQL can delete data from SQL database tables. Finally SQL can modify the database structure itself – create/modify/delete tables and other database objects.

SQL uses set of commands to manipulate the data in relational databases. For example SQL INSERT is used to insert data in database tables. SQL SELECT command is used to retrieve data from one or more database tables. SQL UPDATE is used to modify existing database records.

DDL

Data Definition Language which deals with database schema's and descriptions, of how the data should reside in the database.

• CREATE - To create a database and its objects like (table, index, views, store procedure, function, and triggers)
• ALTER - Alters the structure of the existing 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 which deals with data manipulation and includes most common SQL statements such SELECT, INSERT, UPDATE, DELETE, etc., and it is used to store, modify, retrieve, delete and update data in a database.

• SELECT - Retrieve data from a database.
• INSERT - Insert data into a table.
• UPDATE - Updates existing data within a table.
• DELETE - Delete all records from a database table.
• MERGE - UPSERT operation (insert or update).
• CALL - call a PL/SQL or Java subprogram.
• EXPLAIN PLAN - Interpretation of the data access path.
• LOCK TABLE - Concurrency Control.

DCL

Data Control Language which includes commands such as GRANT and mostly concerned with rights, permissions and other controls of the database system.

• GRANT - Allow users access privileges to the database.
• REVOKE - Withdraw users access privileges given by using the GRANT command.

TCL

Transaction Control Language which deals with a transaction within a database.

• COMMIT - Commits a Transaction.
• ROLLBACK - Rollback a transaction in case of any error occurs.
• SAVEPOINT - To rollback the transaction making points within groups.
• SET TRANSACTION - Specify characteristics of the transaction.