MySQL RAND Function
MySQL has a RAND function that can be invoked to produce random numbers between 0 and 1:
mysql> SELECT RAND( ), RAND( ), RAND( );
+——————+—————–+——————+
| 0.45464584925645 | 0.1824410643265 | 0.54826780459682 |
+——————+—————–+——————+
1 row in set (0.00 sec)
When invoked with an integer argument, RAND( ) uses that value to seed the random number generator. Each time you seed the generator with a given value, RAND( ) will produce a repeatable series of numbers:
mysql> SELECT RAND(1), RAND( ), RAND( );
+——————+——————+——————+
| 0.18109050223705 | 0.75023211143001 | 0.20788908117254 |
+——————+——————+——————+
1 row in set (0.00 sec)
Understanding Order by Rand()
To understand ORDER BY RAND() function, consider an employee_tbl table, which is having the following records:
mysql> SELECT * FROM employee_tbl;
+——+——+————+——————–+
| id | name | work_date | daily_typing_pages |
+——+——+————+——————–+
| 1 | John | 2007-01-24 | 250 |
| 2 | Ram | 2007-05-27 | 220 |
| 3 | Jack | 2007-05-06 | 170 |
| 3 | Jack | 2007-04-06 | 100 |
| 4 | Jill | 2007-04-06 | 220 |
| 5 | Zara | 2007-06-06 | 300 |
| 5 | Zara | 2007-02-06 | 350 |
+——+——+————+——————–+
7 rows in set (0.00 sec)
Now, use the following commands:
mysql> SELECT * FROM employee_tbl ORDER BY RAND();
+——+——+————+——————–+
| id | name | work_date | daily_typing_pages |
+——+——+————+——————–+
| 5 | Zara | 2007-06-06 | 300 |
| 3 | Jack | 2007-04-06 | 100 |
| 3 | Jack | 2007-05-06 | 170 |
| 2 | Ram | 2007-05-27 | 220 |
| 4 | Jill | 2007-04-06 | 220 |
| 5 | Zara | 2007-02-06 | 350 |
| 1 | John | 2007-01-24 | 250 |
+——+——+————+——————–+
7 rows in set (0.01 sec)
mysql> SELECT * FROM employee_tbl ORDER BY RAND();
+——+——+————+——————–+
| id | name | work_date | daily_typing_pages |
+——+——+————+——————–+
| 5 | Zara | 2007-02-06 | 350 |
| 2 | Ram | 2007-05-27 | 220 |
| 3 | Jack | 2007-04-06 | 100 |
| 1 | John | 2007-01-24 | 250 |
| 4 | Jill | 2007-04-06 | 220 |
| 3 | Jack | 2007-05-06 | 170 |
| 5 | Zara | 2007-06-06 | 300 |
+——+——+————+——————–+
7 rows in set (0.00 sec)
Few More Example:
mysql> SELECT RAND(); +-------------------+ | RAND() | +-------------------+ | 0.369500624360052 | +-------------------+ 1 row in set (0.00 sec)
