SQL Reference

SQL

Welcome to SQL

#

Our SQL language reference gives you SQL answers quickly.

We cover important SQL concepts and features -- with examples.

Try the SQL examples live in our online SQL Editor.


What is SQL

SQL stands for Structure Query Language.

SQL is a programming language that is used to manage relational databases.

SQL commands retrieve, create, update, and delete data from a database.

Other commands can be used to control and manage the database.


SQL Commands

SQL commands fall into into 5 catagories:

  • DQL = Data Query Language
  • DML = Data Manipulation Language
  • TCL = Transaction Control Language
  • DDL = Data Definition Language
  • DCL = Data Control Language

Data Query Language (DQL)

#

Data Query Language commands are used to retrieve data from the database. This category has only one command, but it is, by far, the most commonly used query.

  • SELECT - retrieves database records

Data Manipulation Language (DML)

#

Data Manipulation Language commands are used to create, modify or delete data in the database. These commands include:

  • INSERT - adds a single or multiple records in the table
  • UPDATE - modifies an existing record
  • DELETE - removes a record from the database

Transaction Control Language (TCL)

#

Transaction Control Language commands are used to control database transactions. These commands include:

  • COMMIT - commits a transaction
  • ROLLBACK - undo a transaction
  • SAVE - sets a transaction savepoint

Data Definition Language (DDL)

#

Data Definition Language commands are used to define and modify database structures. These commands include:

  • CREATE - creates database and other objects (tables, views, indexes, etc.)
  • DROP - deletes database and other objects
  • ALTER - modifies the database structure
  • TRUNCATE - removes all records from a table

Data Control Language (DCL)

#

Data Control Language commands are used to grant or revoke user rights and permissions. These commands include:

  • GRANT - gives priveleges to a database user
  • REVOKE - removes priveleges from a database user

SQL Process

When SQL Server executes a command, it uses a number of components to interpret the request and determine the optimal way to perform the task. This process includes the following steps:

SQL Process

SQL Server Agent

SQL Server Agent is a service that executes scheduled administrative operations, called jobs, in SQL Server. It is a separate process from the database that can be started and stopped independently.

SQL Server Agent can run a job 1) at a scheduled time, or 2) in response to a certain event, or 3) on demand (i.e. manually).

An example would be backup job that runs every day at 2:00 AM. Before the actual backup operation, perhaps a stored procedure needs to run to ensure that all aggregate data is up-to-date. This can all be part of a single job that consists of one or more tasks.


You may also like

Guides


vsn 3.1