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

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

Calculating Descriptive Statistics shows how to compute descriptive statistics for the entire set of scores in the testscore table. To be more specific, you can use GROUPBY 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 GROUPBY clauses.

Here’s how to calculate 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,
    -> STDDEV_SAMP(score) AS 'std. dev.',
    -> VAR_SAMP(score) AS 'variance'
    -> FROM testscore
    -> GROUP BY age;
+-----+---+------+---------+---------+--------+-----------+----------+
| age | n | sum  | minimum | maximum | mean   | std. dev. | variance |
+-----+---+------+---------+---------+--------+-----------+----------+
|   5 | 4 |   22 |       4 |       7 | 5.5000 |    1.2910 |   1.6667 |
|   6 | 4 |   27 |       4 |       9 | 6.7500 |    2.2174 |   4.9167 |
|   7 | 4 |   30 |       6 |       9 | 7.5000 |    1.2910 |   1.6667 |
|   8 | 4 |   32 |       6 |      10 | 8.0000 |    1.8257 |   3.3333 |
|   9 | 4 |   35 |       7 |      10 | 8.7500 |    1.2583 |   1.5833 |
+-----+---+------+---------+---------+--------+-----------+----------+

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,
    -> STDDEV_SAMP(score) AS 'std. dev.',
    -> VAR_SAMP(score) AS 'variance'
    -> FROM testscore
    -> GROUP BY sex;
+-----+----+------+---------+---------+--------+-----------+----------+
| sex | n  | sum  | minimum | maximum | mean   | std. dev. | variance |
+-----+----+------+---------+---------+--------+-----------+----------+
| M   | 10 |   71 |       4 |       9 | 7.1000 |    1.7920 |   3.2111 |
| F   | 10 |   75 |       4 |      10 | 7.5000 |    1.9579 |   3.8333 |
+-----+----+------+---------+---------+--------+-----------+----------+

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,
    -> STDDEV_SAMP(score) AS 'std. dev.',
    -> VAR_SAMP(score) AS 'variance'
    -> FROM testscore
    -> GROUP BY age, sex;
+-----+-----+---+------+---------+---------+--------+-----------+----------+
| age | sex | n | sum  | minimum | maximum | mean   | std. dev. | variance |
+-----+-----+---+------+---------+---------+--------+-----------+----------+
|   5 | M   | 2 |    9 |       4 |       5 | 4.5000 |    0.7071 |   0.5000 |
|   5 | F   | 2 |   13 |       6 |       7 | 6.5000 |    0.7071 |   0.5000 |
|   6 | M   | 2 |   17 |       8 |       9 | 8.5000 |    0.7071 |   0.5000 |
|   6 | F   | 2 |   10 |       4 |       6 | 5.0000 |    1.4142 |   2.0000 |
|   7 | M   | 2 |   14 |       6 |       8 | 7.0000 |    1.4142 |   2.0000 |
|   7 | F   | 2 |   16 |       7 |       9 | 8.0000 |    1.4142 |   2.0000 |
|   8 | M   | 2 |   15 |       6 |       9 | 7.5000 |    2.1213 |   4.5000 |
|   8 | F   | 2 |   17 |       7 |      10 | 8.5000 |    2.1213 |   4.5000 |
|   9 | M   | 2 |   16 |       7 |       9 | 8.0000 |    1.4142 |   2.0000 |
|   9 | F   | 2 |   19 |       9 |      10 | 9.5000 |    0.7071 |   0.5000 |
+-----+-----+---+------+---------+---------+--------+-----------+----------+