MySQL extracts random data from the table _ MySQL

MySQL extracts random data from the table to achieve random execution. the following statement is found in the manual to complete the task:

SELECT * FROM table_name order by rand () LIMIT 5;

Rand says this in the manual ::

RAND ()

RAND (N)

Returns a random floating point value ranging from 0 to 1.0. If an integer parameter N is specified, it is used as a seed value.

Mysql> select RAND ();

-> 0.5925

Mysql> select RAND (20 );

-> 0.1811

Mysql> select RAND (20 );

-> 0.1811

Mysql> select RAND ();

-> 0.2079

Mysql> select RAND ();

-> 0.7888

You cannot use the column with the RAND () value in an order by clause, because order by will calculate the column multiple times. However, in MySQL3.23, you can do: SELECT * FROM table_name order by rand (), which is helpful for obtaining a data from select * FROM table1, table2 WHERE a = B AND c

Note that an RAND () in a WHERE clause will be re-evaluated every time the WHERE clause is executed.

But after a try, it takes 0.08 sec to execute a record table, which is slower. Then I consulted google and got the following code:

SELECT * FROM table_name AS r1 JOIN (select round (RAND () * (select max (id) FROM table_name) AS id) AS r2 WHERE r1.id> = r2.id order by r1.id asc limit 5;

The execution efficiency requires 0.02 sec. Unfortunately, only mysql. * and above support such subqueries.