13.3 Per-Group Descriptive Statistics
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 |
+-----+-----+---+------+---------+---------+--------+-----------+
|
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