How do I select a fixed number of rows for each group?

Question

Here is some example data in a mysql table

a   b   distance
15  44  250
94  31  250
30  41  250
6   1   250
95  18  250
72  84  500
14  23  500
55  24  500
9         


        

Answer 1

One way would be to use union. Like so:

(SELECT a, b, distance FROM table WHERE distance = 250 LIMIT 2 ORDER BY RAND())
UNION
(SELECT a, b, distance FROM table WHERE distance = 500 LIMIT 2 ORDER BY RAND())
...
ORDER BY distance

I can think of a way of getting one of each with one query using distinct =/, but like I said that would only bring you one.