顯示父表中子記錄少於4個的記錄

Question

我需要顯示來自父表的sql記錄，該記錄的子記錄少於4個（根本沒有子記錄也可以），請使用SQL查詢。

Answer 1

這是查詢：

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

您可以使用子查詢來計算子行的數量：

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