with a as (
select 1 as boardId, '김' as userId from dual union all
select 2 as boardId, '이' as userId from dual union all
select 3 as boardId, '정' as userId from dual union all
select 4 , '박' from dual

),
b as (
select 1 as boardId , '김씨 댓글1' as board from dual union all
select 1, '김씨 댓글2' from dual union all
select 1, '김씨 댓글3' from dual union all
select 4, '박씨 댓글1' from dual union all
select 2, '이씨 댓글1' from dual union all
select 2, '이씨 댓글2' from dual union all
select 2, '이씨 댓글3' from dual union all
select 3, '정씨 댓글1' from dual union all
select 3, '정씨 댓글2' from dual union all
select 3, '정씨 댓글3' from dual union all
select 1, '김씨 댓글4' from dual union all
select 4, '박씨 댓글2' from dual union all
select 4, '박씨 댓글3' from dual union all
select 4, '박씨 댓글4' from dual union all
select 4, '박씨 댓글5' from dual
)
select at.*, bt.CNT from a at, (select boardId, max(CNT) CNT from (select boardId, ROW_NUMBER() OVER( PARTITION BY boardId ORDER BY board) CNT from b) GROUP BY boardId) bt where at.boardId = bt.boardId order by bt.CNT desc  

이렇게 만들어봤는데 mysql에서 될지 모르겠네요... 전 오라클 유저라;;
전 완전초보라서 공부할겸 만들어본거에요 고수분들이 빨리 답글 달아주시길 빌께요