Question

Say, I have a table:

id ext_id param

1    5      0

2    5      1

3    6      0

4    6      0

5    7      1

6    7      1

respectively.

I wanna have a result like:

2 5 1

3 6 0

5 7 1

So that MySQL groups by ext_id and shows an element, that has maximum param value. If all are equal – I need any.

I tried something like:

SELECT * FROM t 
GROUP BY ext_id HAVING param = MAX(param)

but it did not do the trick ((

Answer 1

First you have to find the maximum param for every ext_id, using a group by query. Then you have to join the result of this query again on t, where ext_id matches, and param is max_param:

SELECT t2.*
FROM (
  SELECT ext_id, max(param) as max_param
  FROM t
  GROUP BY ext_id ) t1 inner join t t2
  on t1.ext_id=t2.ext_id and t1.max_param=t2.param
GROUP BY t2.ext_id

Since there could be more than one row with the same maximum param, I’m grouping again by ext_id, MySql allows you to do so, so it will get just one row for each ext_id.