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MySQL Cookbook
book

MySQL Cookbook

by Paul DuBois
October 2002
Intermediate to advanced content levelIntermediate to advanced
1024 pages
27h 26m
English
O'Reilly Media, Inc.
Content preview from MySQL Cookbook

Reversing or Negating Query Conditions

Problem

You know how to write a query to answer a given question; now you want to ask the opposite question.

Solution

Reverse the conditions in the WHERE clause by using negation operators.

Discussion

The WHERE conditions in a query can be negated to ask the opposite questions. The following query determines when users sent mail to themselves:

mysql> SELECT * FROM mail WHERE srcuser = dstuser;
+---------------------+---------+---------+---------+---------+-------+
| t                   | srcuser | srchost | dstuser | dsthost | size  |
+---------------------+---------+---------+---------+---------+-------+
| 2001-05-12 15:02:49 | phil    | mars    | phil    | saturn  |  1048 |
| 2001-05-14 14:42:21 | barb    | venus   | barb    | venus   | 98151 |
| 2001-05-15 07:17:48 | gene    | mars    | gene    | saturn  |  3824 |
| 2001-05-15 08:50:57 | phil    | venus   | phil    | venus   |   978 |
| 2001-05-15 17:35:31 | gene    | saturn  | gene    | mars    |  3856 |
| 2001-05-19 22:21:51 | gene    | saturn  | gene    | venus   | 23992 |
+---------------------+---------+---------+---------+---------+-------+

To reverse this query, to find records where users sent mail to someone other than themselves, change the comparison operator from = (equal to) to != (not equal to):

mysql> SELECT * FROM mail WHERE srcuser != dstuser; +---------------------+---------+---------+---------+---------+---------+ | t | srcuser | srchost | dstuser | dsthost | size | +---------------------+---------+---------+---------+---------+---------+ | 2001-05-11 10:15:08 ...
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Publisher Resources

ISBN: 0596001452Catalog PageErrata