Mysql::错误：子查询返回超过 1 行：

Question

在我的 Rails 应用程序中，我正在使用 find_by_sql() 运行 sql 查询，因为我需要子查询。 如果我执行第一个或第二个查询，这是有效的，但是当我将它们与 AND 添加在一起时，它开始抱怨子查询中的行数超过 1 行。

我希望返回符合条件的所有行（记录）。 这里需要修复/更改什么？ 什么告诉 mysql 我只想要 1 行？

下面是在 Rails 日志中查看的结果 SQL：

Mysql::Error: Subquery returns more than 1 row: select p.* from policies p 
 where exists (select 0 from status_changes sc join statuses s on sc.status_id = s.id
 where sc.policy_id = p.id
 and s.status_category_id = '1'
 and sc.created_at between '2009-03-10' and '2009-03-12')
 or exists
 (select 0 from status_changes sc join statuses s on sc.status_id = s.id
 where sc.created_at in
 (select max(sc2.created_at)
 from status_changes sc2
 where sc2.policy_id = p.id
 and sc2.created_at < '2009-03-10')
 and s.status_category_id = '1'
 and sc.policy_id = p.id) 
AND (select 0 from status_changes sc
 where sc.policy_id = p.id
 and sc.status_id = 7
 and sc.created_at between '2008-12-31' and '2009-03-12')
 or exists
 (select 0 from status_changes sc
 where sc.created_at in
 (select max(sc2.created_at)
 from status_changes sc2
 where sc2.policy_id = p.id
 and sc2.created_at < '2008-12-31')
 and sc.status_id = 7
 and sc.policy_id = p.id)

Answer 1

这一行：

AND (select 0 from status_changes sc

不应该吗

AND exists (select 0 from status_changes sc

Answer 2

据我所知，任何 SQL 服务器都不支持返回超过 1 行的子查询。