How do I select random rows in MySQL?

Question

mytable

pid name field
=== ==== =====
1    A1   0
2    A2   1
3    A3   1
4    A4   0   
5    A5   0

This is my table structure. Here I want to se

Answer 1

I ran a heavy test on this, passed.

(
SELECT * , 0.5 AS ordercol
FROM `mytable`
WHERE `name`IN ( "A2", "A3" )
LIMIT 2
)
UNION (
SELECT * , rand() AS ordercol
FROM `mytable`
WHERE `name` NOT IN ( "A2", "A3" )
LIMIT 2
)
ORDER BY ordercol, `name` IN ( "A2", "A3" ) , `name` ="A3"

This will do the job very well. But to make the result even more random, execute that statement with replacing that 0.5 value in 1st line with a random value chosen by your client application code like mt_rand(0, 1000000) / 1000000 in PHP . Make sure it falls between 0 and 1. But do NOT use mysql function rand() in place of that 0.5 because it will make A2 and A3 apart from each other. The trick is assigning a random value for "ordercol" in all rows but keep it same for A2 and A3

EDIT: I believe we can replace the 0.5 value with a LEFT JOIN even instead of relying on discrete value by PHP, as we replace the first segment of the union, so the whole query becomes:

(
SELECT mt1.* , mt2.ordercol AS ordercol
FROM `mytable` AS mt1
LEFT JOIN (

SELECT RAND( ) AS ordercol
) AS mt2 ON TRUE
WHERE `name`
IN (
"A2", "A3"
)
LIMIT 2
)
UNION (
SELECT * , rand() AS ordercol
FROM `mytable`
WHERE `name` NOT IN ( "A2", "A3" )
LIMIT 2
)
ORDER BY ordercol, `name` IN ( "A2", "A3" ) , `name` ="A3"