随机 select 一半的记录

Question

I'm trying to generate a random sample of half of a table (or some other percentage). The table is small enough that I can use the ORDER BY RAND() LIMIT x approach. I'd like the code to sample 50% of recipients as the table changes size over time. Below was my first attempt but you can't put a subquery in a LIMIT clause. Any ideas?

SELECT
    recipient_id
FROM
    recipient
ORDER BY RAND()
LIMIT (
    /* Find out how many recipients are on half the list */
    SELECT
        COUNT(*) / 2
    FROM
        recipient
);

Answer 1

If you are running MysQL 8.0, you can use window functions:

select *
from (select t.*, ntile(2) over(order by random()) nt from mytable t) t
where nt = 1

In earlier versions, one approach uses user variables:

select t.*
from (
    select t.*, @rn := @rn + 1 rn
    from (select * from mytable order by random()) t
    cross join (select @rn := 0) x
) t
inner join (select count(*) cnt from mytable) c on t.rn <= c.cnt / 2