Selecting two different random entries for each category with mySQL

Question

I have a table which looks something like:

table: Q
---------------------------
|question| scope |  type  |
---------------------------
|  this  |   A   |   1    |
|  that  |   A   |   1    |
|  them  |   A   |   1    |
---------------------------
|  this  |   A   |   2    |
|  that  |   A   |   2    |
|  them  |   A   |   2    |
---------------------------
|  this  |   B   |   1    |
|  that  |   B   |   1    |
|  them  |   B   |   1    |
---------------------------

I need given a scope, I need to extract two entries from each type. If scope is A, a possible solution could be:

---------------------------
|  this  |   A   |   1    |
|  them  |   A   |   1    |
---------------------------
|  that  |   A   |   2    |
|  this  |   A   |   2    |
---------------------------

I'm currently using the following SQL statement:

SELECT tmp.question, tmp.type, tmp.scope
FROM Q
LEFT JOIN (
SELECT * FROM Q ORDER BY RAND( )
)tmp ON ( Q.type = tmp.type AND tmp.scope = 'A' ) 
GROUP BY tmp.type
ORDER BY Q.type

However this only returns one entry per type and for some reason a NULL row.
My question is therefore how to optimize the statement to return two rows and eliminate the NULL one?

Answer 1

you can change rank from 2 to whatever you want to get for each category.

http://www.sqlfiddle.com/#!2/f3946/86

try this:

  SELECT x.question,
       x.scope,
       x.type
  FROM (

    SELECT bp.question, bp.scope, bp.type, 
    CASE WHEN bp.type = @type 
         THEN @rownum := @rownum + 1
         ELSE @rownum := 1
         END AS rank,
    @type := bp.type
FROM (select * from Q order by rand()) bp
JOIN (SELECT @rownum := 0, @type := NULL) r
WHERE bp.scope = 'A'
ORDER BY type
    ) x
 WHERE x.rank <= 2
order by x.type

Note: I used an old answer and improved for randomizing. Old answer I got help is: Select N records for each category and order by X