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RAND(), RAND(N)

Returns a random floating-point value v in the range 0 <= v < 1.0. If a constant integer argument N is specified, it is used as the seed value, which produces a repeatable sequence of column values.

mysql> SELECT RAND();
-> 0.9233482386203
mysql> SELECT RAND(20);
-> 0.15888261251047
mysql> SELECT RAND(20);
-> 0.15888261251047
mysql> SELECT RAND();
-> 0.63553050033332
mysql> SELECT RAND();
-> 0.70100469486881
mysql> SELECT RAND(20);
-> 0.15888261251047
The effect of using a non-constant argument is undefined. As of MySQL 5.0.13, non-constant arguments are disallowed.

To obtain a random integer R in the range i <= R < j, use the expression FLOOR(i + RAND() * (j – i). For example, to obtain a random integer in the range the range 7 <= R < 12, you could use the following statement:

SELECT FLOOR(7 + (RAND() * 5));
You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times. However, you can retrieve rows in random order like this:

mysql> SELECT * FROM tbl_name ORDER BY RAND();
ORDER BY RAND() combined with LIMIT is useful for selecting a random sample from a set of rows:

mysql> SELECT * FROM table1, table2 WHERE a=b AND c<d -> ORDER BY RAND() LIMIT 1000;
Note that RAND() in a WHERE clause is re-evaluated every time the WHERE is executed.

RAND() is not meant to be a perfect random generator, but instead is a fast way to generate ad hoc random numbers which is portable between platforms for the same MySQL version.