显示父表中子记录少于4个的记录

Question

我需要显示来自父表的sql记录，该记录的子记录少于4个（根本没有子记录也可以），请使用SQL查询。

Answer 1

Here's the query:

SELECT
  pt.id, pt.somefield, COUNT(pt.id) as c
FROM
  parenttable pt
LEFT OUTER JOIN
  chiledtable ct
ON
  ct.parenttable_id = pt.id
GROUP BY
  pt.id
HAVING
  c <= 4

Answer 2

You could use a subquery to calculate the number of child rows:

select  *
from    ParentTable as p
left join
        (
        select  parentid
        ,       count(*) as cnt
        from    ChildTable
        group by
                parentid
        ) as c
on      c.parentid = p.id
where   c.cnt <= 3