Finding Smallest or Largest Summary Values

7.15.1 Problem

You want to compute per-group summary values, but display only the smallest or largest of them.

7.15.2 Solution

Add a LIMIT clause to the query.

7.15.3 Discussion

MIN( ) and MAX( ) find the values at the endpoints of a range of values, but if you want to know the extremes of a set of summary values, those functions won't work. The arguments to MIN( ) and MAX( ) cannot be other aggregate functions. For example, you can easily find per-driver mileage totals:

mysql> SELECT name, SUM(miles)
 -> FROM driver_log
 -> GROUP BY name;
+-------+------------+
| name | SUM(miles) |
+-------+------------+
| Ben | 362 |
| Henry | 911 |
| Suzi | 893 |
+-------+------------+

But to select only the record for the driver with the most miles, this doesn't work:

mysql> SELECT name, SUM(miles)
 -> FROM driver_log
 -> GROUP BY name
 -> HAVING SUM(miles) = MAX(SUM(name));
ERROR 1111 at line 1: Invalid use of group function

Instead, order the rows with the largest SUM( ) values first and use LIMIT to select the first record:

mysql> SELECT name, SUM(miles) AS 'total miles'
 -> FROM driver_log
 -> GROUP BY name
 -> ORDER BY 'total miles' DESC LIMIT 1;
+-------+-------------+
| name | total miles |
+-------+-------------+
| Henry | 911 |
+-------+-------------+

An alias is used in the ORDER BY clause because ORDER BY cannot refer directly to aggregate functions, as discussed earlier in Recipe 7.14.

Note that if there is more than one row with the given summary value, this type of query won't tell you that. For example, you might attempt to ascertain the most common initial letter for state names like this:

mysql> SELECT LEFT(name,1) AS letter, COUNT(*) AS count FROM states
 -> GROUP BY letter ORDER BY count DESC LIMIT 1;
+--------+-------+
| letter | count |
+--------+-------+
| M | 8 |
+--------+-------+

But eight state names also begin with N. If you need to know all most-frequent values when there may be more than one of them, a two-query approach will be more useful:

mysql> SELECT LEFT(name,1) AS letter, @max:=COUNT(*) AS count FROM states
 -> GROUP BY letter ORDER BY count DESC LIMIT 1;
mysql> SELECT LEFT(name,1) AS letter, COUNT(*) AS count FROM states
 -> GROUP BY letter HAVING count = @max;
+--------+-------+
| letter | count |
+--------+-------+
| M | 8 |
| N | 8 |
+--------+-------+

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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