Selecting Random Items from a Set of Rows

13.9.1 Problem

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

13.9.2 Solution

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

13.9.3 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.
• "Pick a card, any card" magic tricks that begin with a full deck of cards each time.

If you want to pick more than one item, change the LIMIT argument. For example, to draw five winning entries at random from a table named drawing that contains contest entries, use RAND( ) in combination with LIMIT:

SELECT * FROM drawing ORDER BY RAND( ) LIMIT 5;

A special case occurs when you're picking a single row from a table that you know contains a column with values in the range from 1 to n in unbroken sequence. Under these circumstances, it's possible to avoid performing an ORDER BY operation on the entire table by picking a random number in that range and selecting the matching row:

SET @id = FLOOR(RAND( )*n)+1;
SELECT ... FROM tbl_name WHERE id = @id;

This will be much quicker than ORDER BY RAND( ) LIMIT 1 as the table size increases.