Reversing or Negating Query Conditions

3.10.1 Problem

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

3.10.2 Solution

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

3.10.3 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 | barb | saturn | tricia | mars | 58274 |
| 2001-05-12 12:48:13 | tricia | mars | gene | venus | 194925 |
| 2001-05-13 13:59:18 | barb | saturn | tricia | venus | 271 |
| 2001-05-14 09:31:37 | gene | venus | barb | mars | 2291 |
...

A more complex query using two conditions might ask when people sent mail to themselves on the same machine:

mysql> SELECT * FROM mail WHERE srcuser = dstuser AND srchost = dsthost;
+---------------------+---------+---------+---------+---------+-------+
| t | srcuser | srchost | dstuser | dsthost | size |
+---------------------+---------+---------+---------+---------+-------+
| 2001-05-14 14:42:21 | barb | venus | barb | venus | 98151 |
| 2001-05-15 08:50:57 | phil | venus | phil | venus | 978 |
+---------------------+---------+---------+---------+---------+-------+

Reversing the conditions for this query involves not only changing the = operators to !=, but changing the AND to OR:

mysql> SELECT * FROM mail WHERE srcuser != dstuser OR srchost != dsthost;
+---------------------+---------+---------+---------+---------+---------+
| t | srcuser | srchost | dstuser | dsthost | size |
+---------------------+---------+---------+---------+---------+---------+
| 2001-05-11 10:15:08 | barb | saturn | tricia | mars | 58274 |
| 2001-05-12 12:48:13 | tricia | mars | gene | venus | 194925 |
| 2001-05-12 15:02:49 | phil | mars | phil | saturn | 1048 |
| 2001-05-13 13:59:18 | barb | saturn | tricia | venus | 271 |
...

You may find it easier just to put the entire original expression in parentheses and negate the whole thing with NOT:

mysql> SELECT * FROM mail WHERE NOT (srcuser = dstuser AND srchost = dsthost);
+---------------------+---------+---------+---------+---------+---------+
| t | srcuser | srchost | dstuser | dsthost | size |
+---------------------+---------+---------+---------+---------+---------+
| 2001-05-11 10:15:08 | barb | saturn | tricia | mars | 58274 |
| 2001-05-12 12:48:13 | tricia | mars | gene | venus | 194925 |
| 2001-05-12 15:02:49 | phil | mars | phil | saturn | 1048 |
| 2001-05-13 13:59:18 | barb | saturn | tricia | venus | 271 |
...

3.10.4 See Also

If a column involved in a condition may contain NULL values, reversing the condition is a little trickier. See Recipe 3.13 for details.

Using the mysql Client Program

Writing MySQL-Based Programs

Record Selection Techniques

Working with Strings

Working with Dates and Times

Sorting Query Results

Generating Summaries

Modifying Tables with ALTER TABLE

Obtaining and Using Metadata

Importing and Exporting Data

Generating and Using Sequences

Using Multiple Tables

Statistical Techniques

Handling Duplicates

Performing Transactions

Introduction to MySQL on the Web

Incorporating Query Resultsinto Web Pages

Processing Web Input with MySQL

Using MySQL-Based Web Session Management

Appendix A. Obtaining MySQL Software

Appendix B. JSP and Tomcat Primer

Appendix C. References



MySQL Cookbook
MySQL Cookbook
ISBN: 059652708X
EAN: 2147483647
Year: 2005
Pages: 412
Authors: Paul DuBois

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