WHERE Clauses and Column Aliases

3.8.1 Problem

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

3.8.2 Solution

Sorry, you cannot.

3.8.3 Discussion

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

mysql> SELECT t, srcuser, dstuser, size/1024 AS kilobytes
 -> FROM mail WHERE kilobytes > 500;
ERROR 1054 at line 1: Unknown column 'kilobytes' in 'where clause'

The error occurs because aliases name output columns, whereas a WHERE clause operates on input columns to determine which rows to select for output. To make the query 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 |
+---------------------+---------+---------+-----------+
| 2001-05-14 17:03:01 | tricia | phil | 2338.36 |
| 2001-05-15 10:25:52 | gene | tricia | 975.13 |
+---------------------+---------+---------+-----------+