7.11.1 Problem
You want to know whether table values are unique.
7.11.2 Solution
Use HAVING in conjunction with COUNT( ).
7.11.3 Discussion
You can use HAVING to find unique values in situations to which DISTINCT does not apply. DISTINCT eliminates duplicates, but doesn't show which values actually were duplicated in the original data. HAVING can tell you which values were unique or non-unique.
The following queries show the days on which only one driver was active, and the days on which more than one driver was active. They're based on using HAVING and COUNT( ) to determine which trav_date values are unique or non-unique:
mysql> SELECT trav_date, COUNT(trav_date) -> FROM driver_log -> GROUP BY trav_date -> HAVING COUNT(trav_date) = 1; +------------+------------------+ | trav_date | COUNT(trav_date) | +------------+------------------+ | 2001-11-26 | 1 | | 2001-11-27 | 1 | | 2001-12-01 | 1 | +------------+------------------+ mysql> SELECT trav_date, COUNT(trav_date) -> FROM driver_log -> GROUP BY trav_date -> HAVING COUNT(trav_date) > 1; +------------+------------------+ | trav_date | COUNT(trav_date) | +------------+------------------+ | 2001-11-29 | 3 | | 2001-11-30 | 2 | | 2001-12-02 | 2 | +------------+------------------+
This technique works for combinations of values, too. For example, to find message sender/recipient pairs between whom only one message was sent, look for combinations that occur only once in the mail table:
mysql> SELECT srcuser, dstuser -> FROM mail -> GROUP BY srcuser, dstuser -> HAVING COUNT(*) = 1; +---------+---------+ | srcuser | dstuser | +---------+---------+ | barb | barb | | gene | tricia | | phil | barb | | tricia | gene | | tricia | phil | +---------+---------+
Note that this query doesn't print the count. The first two examples did so, to show that the counts were being used properly, but you can use a count in a HAVING clause without including it in the output column list.
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