MySQL Cookbook

Assigning Ranks

Problem

You want to assign ranks to a set of values.

Solution

Decide on a ranking method, then put the values in the desired order and apply the method to them.

Discussion

Some kinds of statistical tests require assignment of ranks. I’ll describe three ranking methods and show how each can be implemented using SQL variables. The examples assume that a table t contains the following scores, which are to be ranked with the values in descending order:

mysql> SELECT score FROM t ORDER BY score DESC;
+-------+
| score |
+-------+
|     5 |
|     4 |
|     4 |
|     3 |
|     2 |
|     2 |
|     2 |
|     1 |
+-------+

One type of ranking simply assigns each value its row number within the ordered set of values. To produce such rankings, keep track of the row number and use it for the current rank:

mysql> SET @rownum := 0;
mysql> SELECT @rownum := @rownum + 1 AS rank, score
    -> FROM t ORDER BY score DESC;
+------+-------+
| rank | score |
+------+-------+
|    1 |     5 |
|    2 |     4 |
|    3 |     4 |
|    4 |     3 |
|    5 |     2 |
|    6 |     2 |
|    7 |     2 |
|    8 |     1 |
+------+-------+

That kind of ranking doesn’t take into account the possibility of ties (instances of values that are the same). A second ranking method does so by advancing the rank only when values change:

mysql> SET @rank = 0, @prev_val = NULL;
mysql> SELECT @rank := IF(@prev_val=score,@rank,@rank+1) AS rank,
    -> @prev_val := score AS score
    -> FROM t ORDER BY score DESC; +------+-------+ | rank | score | +------+-------+ | 1 | 5 | | 2 | 4 | | 2 | 4 | | 3 | 3 | | 4 | 2 | | 4 ...