Alias (SQL)

An alias is a feature of SQL that is supported by most, if not all, relational database management systems (RDBMSs). Aliases provide database administrators, as well as other database users, with the ability to reduce the amount of code required for a query, and to make queries simpler to understand. In addition, aliasing can be used as an obfuscation technique to protect the real names of database fields.

In SQL, you can alias tables and columns. A table alias is also called a correlation name.^[1] A programmer can use an alias temporarily assign another name to a table or column for the duration of a SELECT query. Assigning an alias does not actually rename the column or table. This is often useful when either tables or their columns have very long or complex names. An alias name could be anything, but usually it is kept short. For example, it might be common to use a table alias such as "pi" for a table named "price_information".

The general syntax of an alias is SELECT * FROM table_name [AS] alias_name. Note that the AS keyword is completely optional and is usually kept for readability purposes. Here is some sample data that the queries below will be referencing:

Department Table

DepartmentID	DepartmentName
31	Sales
33	Engineering
34	Clerical
35	Marketing

Using a table alias:

 SELECT D.DepartmentName FROM Department AS D

We can also write the same query like this (Note that the AS clause is omitted this time):

 SELECT D.DepartmentName FROM Department D

A column alias is similar:

 SELECT d.DepartmentId AS Id, d.DepartmentName AS Name FROM Department d

In the returned result sets, the data shown above would be returned, with the only exception being "DepartmentID" would show up as "Id", and "DepartmentName" would show up as "Name".

Also, if only one table is being selected and the query is not using table joins, it is permissible to omit the table name or table alias from the column name in the SELECT statement. Example as follows:

 SELECT DepartmentId AS Id, DepartmentName AS Name FROM Department d

References

1. ANSI Standard SQL - Foundation Document - Date: 2010-10-14