MySQL Cookbook, 2nd Edition

Summaries and NULL Values

Problem

You’re summarizing a set of values that may include NULL values and you need to know how to interpret the results.

Solution

Understand how aggregate functions handle NULL values.

Discussion

Most aggregate functions ignore NULL values. Suppose that you have a table expt that records experimental results for subjects who are to be given four tests each and that lists the test score as NULL for those tests that have not yet been administered:

mysql>SELECT subject, test, score FROM expt ORDER BY subject, test;
+---------+------+-------+
| subject | test | score |
+---------+------+-------+
| Jane    | A    |    47 |
| Jane    | B    |    50 |
| Jane    | C    |  NULL |
| Jane    | D    |  NULL |
| Marvin  | A    |    52 |
| Marvin  | B    |    45 |
| Marvin  | C    |    53 |
| Marvin  | D    |  NULL |
+---------+------+-------+

By using a GROUP BY clause to arrange the rows by subject name, the number of tests taken by each subject, as well as the total, average, lowest, and highest scores can be calculated like this:

mysql>SELECT subject,
    -> COUNT(score) AS n,
    -> SUM(score) AS total,
    -> AVG(score) AS average,
    -> MIN(score) AS lowest,
    -> MAX(score) AS highest
    -> FROM expt GROUP BY subject;
+---------+---+-------+---------+--------+---------+
| subject | n | total | average | lowest | highest |
+---------+---+-------+---------+--------+---------+
| Jane    | 2 |    97 | 48.5000 |     47 |      50 |
| Marvin  | 3 |   150 | 50.0000 |     45 |      53 |
+---------+---+-------+---------+--------+---------+

You can see from the results in the column ...