How MySQL takes random data out of a table
I used to discuss this problem in the group, which is interesting. mysql syntax is fun.
They used to want to use PHP to implement random, but take out more than two times like to make more than one query.
Turn over the manual, find the following statement, you can complete the task
SELECT * FROM table_name ORDER by RAND () LIMIT 5;
That's what Rand says in his handbook:
RAND ()
RAND (N)
Returns a random floating-point value within a range of 0 to 1.0. If an integer parameter n is specified, it is used as the 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 a column with the rand () value in an ORDER BY clause because the order by will repeat the computed column multiple times. However in MySQL3.23, you can do: SELECT * FROM table_name ORDER by RAND (), which is advantageous to get a from SELECT * from Table1,table2 WHERE a=b and C A random sample of the collection of
But I tried, 8,000 records of the table, 0.08 sec is required to execute once. It's a little slow.
later consulted Google, get the following code
SELECT *
from table_name as R1 JOIN
(SELECT R Ound (RAND () *
(SELECT MAX (ID)
from table_name)) as ID)
as R2
WHERE r1.id >= r2.id
Orde R by R1.id ASC
LIMIT 5;
Execution Efficiency requires 0.02 sec. Unfortunately, only MySQL 4.1.* above supports such subqueries.
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