Mysql group by two columns and pick the maximum value of third column

Question

I have a table that has user_id, item_id and interaction_type as columns. interaction_type could be 0, 1,2,3,4 or 5. However, for some user_id and item_id pairs, we might have m

Answer 1

Your query is fine. The reason you are getting 2000 rows is because you are getting one row for every unique pair of values user_id, item_id.

If you want to see the interaction types going into each row then use:

select user_id, item_id, max(interaction_type) as max_type,
       group_concat(distinct interaction_type) as interaction_types,
       count(*) as cnt
from mytable
group by user_id, item_id;

It occurs to me that you want all rows with the maximum interaction type. If so, calculate the maximum and then find all rows that match that value:

select t.*
from mytable t cross join
     (select max(interaction_type) as maxit from mytable) x
     on x.maxit = t.interaction_type;

No group by is needed for this query.