Column Aliases

The AS keyword lets you rename a column in the output of a query, without changing anything about the underlying table. This is especially useful for computed columns, which otherwise show up with an unreadable, database-generated name.

Renaming a column with AS

A simple alias

SQL

SELECT first_name AS "First Name",
       last_name  AS "Last Name"
FROM employees;

Many databases also let you drop the word AS entirely and just put the alias directly after the expression — both forms mean the same thing:

AS is often optional

SQL

SELECT first_name "First Name", last_name "Last Name"
FROM employees;

Making expressions readable

Aliases are most valuable on computed columns, where the database would otherwise generate an unhelpful default name like ?column? or an entire copy of the expression:

Aliasing a computed column

SQL

SELECT product_name,
       price * quantity AS total_price
FROM order_items;

Quoting aliases with spaces

If an alias contains spaces or mixed case you want preserved, it needs to be quoted. Standard SQL and PostgreSQL use double quotes ("First Name"), while MySQL by default uses backticks (`First Name`). SQL Server accepts either double quotes or square brackets ([First Name]). Unquoted aliases generally cannot contain spaces.

Where you can (and can’t) use an alias

Aliases usually can't be used in WHERE

Because of SQL’s logical execution order, WHERE runs *before* SELECT computes its column list — so at the point WHERE is evaluated, a SELECT alias does not exist yet. A query like this will typically fail with an “unknown column” error:

This usually does NOT work

SQL

SELECT price * quantity AS total_price
FROM order_items
WHERE total_price > 100;  -- error in most databases

To filter on that value, repeat the expression in WHERE instead:

Repeat the expression in WHERE

SQL

SELECT price * quantity AS total_price
FROM order_items
WHERE price * quantity > 100;

ORDER BY, on the other hand, runs *after* SELECT in the logical order, so it can freely reference a column alias:

Aliases work fine in ORDER BY

SQL

SELECT price * quantity AS total_price
FROM order_items
ORDER BY total_price DESC;