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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

Per-Group Descriptive Statistics

Problem

You want to produce descriptive statistics for each subgroup of a set of observations.

Solution

Use aggregate functions, but employ a GROUP BY clause to arrange observations into the appropriate groups.

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. ...
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Publisher Resources

ISBN: 0596001452Catalog PageErrata