MySQL Cookbook

Selecting Random Items from a Set of Rows

Problem

You want to pick an item or items randomly from a set of values.

Solution

Randomize the values, then pick the first one (or the first few, if you need more than one).

Discussion

When a set of items is stored in MySQL, you can choose one at random as follows:

• Select the items in the set in random order, using ORDER BY RAND( ) as described in Recipe 13.8.

• Add LIMIT 1 to the query to pick the first item.

For example, a simple simulation of tossing a die can be performed by creating a die table containing rows with values from 1 to 6 corresponding to the six faces of a die cube, then picking rows from it at random:

mysql> SELECT n FROM die ORDER BY RAND( ) LIMIT 1;
+------+
| n    |
+------+
|    6 |
+------+
mysql> SELECT n FROM die ORDER BY RAND( ) LIMIT 1;
+------+
| n    |
+------+
|    4 |
+------+
mysql> SELECT n FROM die ORDER BY RAND( ) LIMIT 1;
+------+
| n    |
+------+
|    5 |
+------+
mysql> SELECT n FROM die ORDER BY RAND( ) LIMIT 1;
+------+
| n    |
+------+
|    4 |
+------+

As you repeat this operation, you pick a random sequence of items from the set. This is a form of selection with replacement: An item is chosen from a pool of items, then returned to the pool for the next pick. Because items are replaced, it’s possible to pick the same item multiple times when making successive choices this way. Other examples of selection with replacement include:

• Selecting a banner ad to display on a web page

• Picking a row for a “quote of the day” application. ...