MySQL Cookbook

Generating Frequency Distributions

Problem

You want to know the frequency of occurrence for each value in a table.

Solution

Derive a frequency distribution that summarizes the contents of your dataset.

Discussion

A common application for per-group summary techniques is to generate a breakdown of the number of times each value occurs. This is called a frequency distribution. For the testscore table, the frequency distribution looks like this:

mysql> SELECT score, COUNT(score) AS occurrence
    -> FROM testscore GROUP BY score;
+-------+------------+
| score | occurrence |
+-------+------------+
|     4 |          2 |
|     5 |          1 |
|     6 |          4 |
|     7 |          4 |
|     8 |          2 |
|     9 |          5 |
|    10 |          2 |
+-------+------------+

If you express the results in terms of percentages rather than as counts, you produce a relative frequency distribution. To break down a set of observations and show each count as a percentage of the total, use one query to get the total number of observations, and another to calculate the percentages for each group:

mysql> SELECT @n := COUNT(score) FROM testscore;
mysql> SELECT score, (COUNT(score)*100)/@n AS percent
    -> FROM testscore GROUP BY score;
+-------+---------+
| score | percent |
+-------+---------+
|     4 |      10 |
|     5 |       5 |
|     6 |      20 |
|     7 |      20 |
|     8 |      10 |
|     9 |      25 |
|    10 |      10 |
+-------+---------+

The distributions just shown summarize the number of values for individual scores. However, if the dataset contains a large number of distinct values and you want a distribution that shows only a small number ...