MYSQL SELECT DISTINCT记录分组

Question

I need to select "four distinct random records" from specific four categories and then order by ascending.

I tried to select sixteen random records and then I have grouped them by category_id

Query:

SELECT * FROM
(
    SELECT 
        id, 
        category_id, 
        description, 
        RAND() AS rnd
    FROM questions
    ORDER BY rnd
    LIMIT 16
) AS temp
GROUP BY temp.category_id
LIMIT 4

Results in some moment :

id      category_id     description     rnd
--------------------------------------------------------------
224         1           Question 7      0.004305024635330797
293         2           Question 10     0.006966075866451558
601         3           Question 2      0.001877430828174046
958         4           Question 54     0.0065207639769844375

Results in other moment :

id      category_id     description     rnd
--------------------------------------------------------------
230         1           Question 2      0.01622675640157122
310         2           Question 21     0.005430353810480194
159         4           Question 17     0.021778853630441106

The problem is that not always show the four categories I need to fix this query, up to now I can't find the real solution.
I need your help !
Thanks in advance !

Answer 1

The key is to choose the categories first and then go back to the original data:

select q.*
from (select category_id, substring_index(group_concat(id order by rand()), ',', 1) as id
      from questions
      group by category_id
      order by rand()
      limit 4
     ) c4 join
     questions q
     on q.id = c4.id
order by category_id

There are other ways to do this, such as by using a bunch of union all statements. But this is general and makes it easy to change the number of categories.