MySQL Cookbook, 2nd Edition

WHERE Clauses and Column Aliases

Problem

You want to refer to a column alias in a WHERE clause.

Solution

Sorry, you cannot. But there is a workaround.

Discussion

You cannot refer to column aliases in a WHERE clause. Thus, the following statement is illegal:

mysql>SELECT t, srcuser, dstuser, size/1024 AS kilobytes
    -> FROM mail WHERE kilobytes > 500;
ERROR 1054 (42S22): Unknown column 'kilobytes' in 'where clause'

The error occurs because an alias names an output column, whereas a WHERE clause operates on input columns to determine which rows to select for output. To make the statement legal, replace the alias in the WHERE clause with the column or expression that the alias represents:

mysql>SELECT t, srcuser, dstuser, size/1024 AS kilobytes
    -> FROM mail WHERE size/1024 > 500;
+---------------------+---------+---------+-----------+
| t                   | srcuser | dstuser | kilobytes |
+---------------------+---------+---------+-----------+
| 2006-05-14 17:03:01 | tricia  | phil    | 2338.3613 |
| 2006-05-15 10:25:52 | gene    | tricia  | 975.1289  |
+---------------------+---------+---------+-----------+