3.9 Displaying Comparisons to Find Out How Something Works
3.9.1 Problem
You're
curious about how a comparison in a WHERE clause
works. Or perhaps about why it doesn't seem to be
working.
3.9.2 Solution
Display the result of the comparison to get more information about
it. This is a useful diagnostic or debugging technique.
3.9.3 Discussion
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 | 271 | 0 |
...
This technique of displaying comparison results is particularly
useful for writing queries that check how a test works without using
a table:
mysql> SELECT 'a' = 'A';
+-----------+
| 'a' = 'A' |
+-----------+
| 1 |
+-----------+
This query result tells you that string comparisons are not by
default case sensitive, which is a useful thing to know.
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