How to select top 3 values from each group in a table with SQL which have duplicates [duplicate]

Answer 1

I was going to downvote the question. However, I realized that it might really be asking for a cross-database solution.

Assuming you are looking for a database independent way to do this, the only way I can think of uses correlated subqueries (or non-equijoins). Here is an example:

select distinct t.personid, val, rank
from (select t.*,
             (select COUNT(distinct val) from t t2 where t2.personid = t.personid and t2.val >= t.val
             ) as rank
      from t
     ) t
where rank in (1, 2, 3)

However, each database that you mention (and I note, Hadoop is not a database) has a better way of doing this. Unfortunately, none of them are standard SQL.

Here is an example of it working in SQL Server:

with t as (
      select 1 as personid, 5 as val union all
      select 1 as personid, 6 as val union all
      select 1 as personid, 6 as val union all
      select 1 as personid, 7 as val union all
      select 1 as personid, 8 as val
     )
select distinct t.personid, val, rank
from (select t.*,
             (select COUNT(distinct val) from t t2 where t2.personid = t.personid and t2.val >= t.val
             ) as rank
      from t
     ) t
where rank in (1, 2, 3);