MySQL Cookbook, 2nd Edition

Debugging Comparison Expressions

Problem

You’re curious about how a comparison in a WHERE clause works. Or perhaps about why it doesn’t seem to be working.

Solution

Display the result of the comparison to get more information about it. This is a useful diagnostic or debugging technique.

Discussion

Normally, you put comparison operations in the WHERE clause of a query and use them to determine which rows to display:

mysql>SELECT * FROM mail WHERE srcuser < 'c' AND size > 5000;
+---------------------+---------+---------+---------+---------+-------+
| t                   | srcuser | srchost | dstuser | dsthost | size  |
+---------------------+---------+---------+---------+---------+-------+
| 2006-05-11 10:15:08 | barb    | saturn  | tricia  | mars    | 58274 |
| 2006-05-14 14:42:21 | barb    | venus   | barb    | venus   | 98151 |
+---------------------+---------+---------+---------+---------+-------+

But sometimes it’s desirable to see the result of the comparison itself (for example, if you’re not sure that the comparison is working the way you expect it to). To do this, just remove the WHERE clause, and put the comparison expression in the output column list, perhaps also including the values that you’re comparing:

mysql>SELECT srcuser, srcuser < 'c', size, size > 5000 FROM mail; +---------+---------------+---------+-------------+ | srcuser | srcuser < 'c' | size | size > 5000 | +---------+---------------+---------+-------------+ | barb | 1 | 58274 | 1 | | tricia | 0 | 194925 | 1 | | phil | 0 | 1048 | 0 | | barb | 1 | 271 ...