mysql - 显示父表中子记录少于 4 的记录

Question

我需要从父表中显示我的 sql 中子记录少于 4 的记录（根本没有子记录也可以），请需要 SQL 查询。

Answer 1

这是查询：

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

您可以使用子查询来计算子行数：

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