mysql - 加入派生表时列不能为空错误

Question

这是我的sql查询：

select
  t1.id, t1.text as text, l.likes as likecount
from
  table1 as t1 left join (select feed_id, count(user_id) as likes
                          from likes
                          where feed_id=12345) l
  on t1.id=l.feed_id where t1.id=12345

此查询仅获取一个特定条目以及有多少人喜欢该条目。在此示例中，条目号为 12345。当条目至少有 1 个类似时，此查询工作正常。但是，当我对没有喜欢的条目运行此查询时，它给出的错误是列提要不能为空。

此错误的原因是内部查询返回了 feed_id 为 null 的行，并且喜欢 0，这在连接表时产生了问题。另一个有趣的因素是，这个东西在我的本地服务器上工作文件，但不在我的实时服务器上。

Answer 1

您只能使用一个 WHERE 子句：

select
  t1.id, t1.text as text, l.likes as likecount
from
  table1 as t1 left join (select feed_id, count(user_id) as likes
                          from likes
                          group by feed_id) l
  on t1.id=l.feed_id
where t1.id=12345

您的子查询中不需要它，但您需要使用 GROUP BY 子句。

但是您可能可以使用以下方法简化上面的查询：

SELECT
  t1.id, t1.text, COUNT(DISTINCT likes.user_id) likecount
FROM
  table1 as t1 LEFT JOIN likes
  ON t1.id = likes.feed_id
WHERE
  t1.id = 12345

Answer 2

尝试添加group by feed_id到您的内部查询：

select
  t1.id, t1.text as text, l.likes as likecount
from
  table1 as t1 left join (select feed_id, count(user_id) as likes
                          from likes
                          where feed_id=12345
                          group by feed_id) l
  on t1.id=l.feed_id where t1.id=12345