select n random rows per specified group

Question

I am struggling to find an optimal solution for the following problem. Suppose I have a table \'Table\' like this:

id    name    report_id
 1    name1    1
          


        

Answer 1

Try this:

  SELECT
    id, 
    name,
    report_id
  FROM
  (
    SELECT id,
        report_id,
        name,
        CASE WHEN @name != name THEN @rn := 1 ELSE @rn := @rn + 1 END rn,
        @name:=name
     FROM (SELECT * FROM tbl ORDER BY RAND()) a,
        (SELECT @rn:=0, @name := NULL) r
     ORDER BY name
  ) s
  WHERE rn <= 10;

SQL FIDDLE DEMO

Answer 2

SQLFiddle demo

select ID,NAME,REPORT_ID
from
(
select *, @row:=if(name=@name,@row,0)+1 as rn, @name:=name from 
(select *,RAND() as trand from t) t1,
(select @row:=0,@name:='') tm2 
order by name,trand
) t2
where rn<=10

Answer 3

This doesn't work in mysql, but in psql you can use partition by

select name,report_id from
(select name,report_id,row_number()
over
(partition by name order by random())
as rn from Table) a
where rn<=10

I had this same question and found this answer from a colleague.