13.3.1 Problem
You want to produce descriptive statistics for each subgroup of a set of observations.
13.3.2 Solution
Use aggregate functions, but employ a GROUP BY clause to arrange observations into the appropriate groups.
13.3.3 Discussion
The preceding section shows how to compute descriptive statistics for the entire set of scores in the testscore table. To be more specific, you can use GROUP BY to divide the observations into groups and calculate statistics for each of them. For example, the subjects in the testscore table are listed by age and sex, so it's possible to calculate similar statistics by age or sex (or both) by application of appropriate GROUP BY clauses.
By age:
mysql> SELECT age, COUNT(score) AS n, -> SUM(score) AS sum, -> MIN(score) AS minimum, -> MAX(score) AS maximum, -> AVG(score) AS mean, -> STD(score) AS 'std. dev.' -> FROM testscore -> GROUP BY age; +-----+---+------+---------+---------+--------+-----------+ | age | n | sum | minimum | maximum | mean | std. dev. | +-----+---+------+---------+---------+--------+-----------+ | 5 | 4 | 22 | 4 | 7 | 5.5000 | 1.1180 | | 6 | 4 | 27 | 4 | 9 | 6.7500 | 1.9203 | | 7 | 4 | 30 | 6 | 9 | 7.5000 | 1.1180 | | 8 | 4 | 32 | 6 | 10 | 8.0000 | 1.5811 | | 9 | 4 | 35 | 7 | 10 | 8.7500 | 1.0897 | +-----+---+------+---------+---------+--------+-----------+
By sex:
mysql> SELECT sex, COUNT(score) AS n, -> SUM(score) AS sum, -> MIN(score) AS minimum, -> MAX(score) AS maximum, -> AVG(score) AS mean, -> STD(score) AS 'std. dev.' -> FROM testscore -> GROUP BY sex; +-----+----+------+---------+---------+--------+-----------+ | sex | n | sum | minimum | maximum | mean | std. dev. | +-----+----+------+---------+---------+--------+-----------+ | M | 10 | 71 | 4 | 9 | 7.1000 | 1.7000 | | F | 10 | 75 | 4 | 10 | 7.5000 | 1.8574 | +-----+----+------+---------+---------+--------+-----------+
By age and sex:
mysql> SELECT age, sex, COUNT(score) AS n, -> SUM(score) AS sum, -> MIN(score) AS minimum, -> MAX(score) AS maximum, -> AVG(score) AS mean, -> STD(score) AS 'std. dev.' -> FROM testscore -> GROUP BY age, sex; +-----+-----+---+------+---------+---------+--------+-----------+ | age | sex | n | sum | minimum | maximum | mean | std. dev. | +-----+-----+---+------+---------+---------+--------+-----------+ | 5 | M | 2 | 9 | 4 | 5 | 4.5000 | 0.5000 | | 5 | F | 2 | 13 | 6 | 7 | 6.5000 | 0.5000 | | 6 | M | 2 | 17 | 8 | 9 | 8.5000 | 0.5000 | | 6 | F | 2 | 10 | 4 | 6 | 5.0000 | 1.0000 | | 7 | M | 2 | 14 | 6 | 8 | 7.0000 | 1.0000 | | 7 | F | 2 | 16 | 7 | 9 | 8.0000 | 1.0000 | | 8 | M | 2 | 15 | 6 | 9 | 7.5000 | 1.5000 | | 8 | F | 2 | 17 | 7 | 10 | 8.5000 | 1.5000 | | 9 | M | 2 | 16 | 7 | 9 | 8.0000 | 1.0000 | | 9 | F | 2 | 19 | 9 | 10 | 9.5000 | 0.5000 | +-----+-----+---+------+---------+---------+--------+-----------+
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