MySQL select 10 random rows from 600K rows fast

Question

How can I best write a query that selects 10 rows randomly from a total of 600k?

Answer 1

The following should be fast, unbiased and independent of id column. However it does not guarantee that the number of rows returned will match the number of rows requested.

SELECT *
FROM t
WHERE RAND() < (SELECT 10 / COUNT(*) FROM t)

Explanation: assuming you want 10 rows out of 100 then each row has 1/10 probability of getting SELECTed which could be achieved by WHERE RAND() < 0.1. This approach does not guarantee 10 rows; but if the query is run enough times the average number of rows per execution will be around 10 and each row in the table will be selected evenly.