Question

I found a SQL statement to the effect of:

SELECT * FROM Users x

My question is: what is x? I have never seen this before.

Thanks.

Answer 1

It’s an alias. The AS keyword is optional and has been left out, but it is the same as:

SELECT * FROM Users AS x

This means you can (in some implementations of SQL, SQL Server being one of them, must) use x in the rest of the query to refer back to the table Users specified here. For example:

SELECT x.MyColumn
FROM Users x
WHERE x.AnotherColumn = 42

There are three general use cases for aliases:

1. Readability. For long table names or when the name will be used many times, it can improve readability. For example, imagine the following without the alias:

   SELECT x.SomeColumn, x.SomeOtherColumn, x.AThirdColumn
   FROM [my crAzy Table Name with spaces in it] x
   WHERE x.AnotherColumn = 42
   

2. Disambiguation. Often used for self-joins, note the use of the same table twice. You must use an alias to differentiate the two instances of the Users table:

   SELECT x.SomeColumn, COUNT(y.SomeColumn)
   FROM Users x
   INNER JOIN Users y ON x.SomeOtherColumn < y.SomeOtherColumn
   GROUP BY x.SomeColumn
   

3. Sub-queries in a FROM or JOIN clause (also called derived tables) must have a name. This is done by specifying an alias:

   SELECT x.SomeColumn
   FROM
   (
       SELECT SomeColumn
       FROM Users
   ) x