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 ...
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