Using an Alias in SQL Calculations

Simply wrap your reused alias with (SELECT alias) :

SELECT 10 AS my_num,        (SELECT my_num) * 5 AS another_numberFROM table

You'll need to use a subselect to use that aliases that way

SELECT my_num*5 AS another_number FROM(    SELECT 10 AS my_num FROM table) x

Aliases in sql are not like variables in a programming language. Aliases can only be referenced again at certain points (particularly in GROUP BY and HAVING clauses). But you can't reuse an alias in the SELECT clause. So you can use a derived query (such as suggested by Rubens Farias) which lets you basically rename your columns and/or name any computed columns.

Or you could use a VIEW if your formulas are generally fixed

CREATE VIEW table10 AS SELECT 10 AS my_num FROM table;SELECT my_num * 5 AS another_number FROM table10;

I believe that will be slightly faster than using a derived query but it probably depends a lot on your real query.

Or you could just duplicate the work:

SELECT 10 AS my_num, 10 * 5 AS another_number FROM table;

Which might be convenient in something like php/perl:

my $my_num = 10;my $query = "SELECT $my_num AS my_num, $my_num * 5 AS another_number FROM table";