October 2002
Intermediate to advanced
1024 pages
27h 26m
English
You’re
curious about how a comparison in a WHERE clause
works. Or perhaps about why it doesn’t seem to be
working.
Display the result of the comparison to get more information about it. This is a useful diagnostic or debugging technique.
Normally you put comparison operations in the
WHERE clause of a query and use them to determine
which records to display:
mysql> SELECT * FROM mail WHERE srcuser < 'c' AND size > 5000;
+---------------------+---------+---------+---------+---------+-------+
| t | srcuser | srchost | dstuser | dsthost | size |
+---------------------+---------+---------+---------+---------+-------+
| 2001-05-11 10:15:08 | barb | saturn | tricia | mars | 58274 |
| 2001-05-14 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 put the comparison expression in the output column list, perhaps including the values that you’re comparing as well:
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 | ...