mysql - 执行查询但没有显示数据 - MySQL

Question

我基本上是在尝试提取某个机构下的某个政策的概要，该机构具有某种政策类型，而且它必须是最新的政策。

我写了一个 SQL 查询，执行时没有错误，但是当应该至少检索一个数据时它没有提取任何数据。

下面是我的查询:

select synopsis
from policy p
     left outer join institution_has_policy ihp
          on p.policy_id = ihp.policy_id
     left outer join institution i 
          on i.institution_id = ihp.institution_id   
where p.policy_type_id = 5  
      and i.institution_name = 'SSgA Funds Management, Inc.'
      and current_as_of_date = (select max(current_as_of_date) from policy p);

我的sql语句是否正确？

Answer 1

这是您的查询:

select synopsis
from policy p left outer join
     institution_has_policy ihp
     on p.policy_id = ihp.policy_id left outer join
     institution i 
     on i.institution_id = ihp.institution_id   
where p.policy_type_id = 5 and
      i.institution_name = 'SSgA Funds Management, Inc.'
      current_as_of_date = (select max(current_as_of_date) from policy p);

未返回任何数据，因为最大截至日期与政策和机构名称不对应。如果你只想返回一行，那么你可以使用 order by 和 limit 来获取最新的:

select synopsis
from policy p left outer join
     institution_has_policy ihp
     on p.policy_id = ihp.policy_id left outer join
     institution i 
     on i.institution_id = ihp.institution_id   
where p.policy_type_id = 5 and
      i.institution_name = 'SSgA Funds Management, Inc.'
order by current_as_of_date desc
limit 1;