MySQL 查询 - where 子句中的未知列 - 执行 WHERE 子句时可能尚未确定列值

Question

 SELECT area_id, 
           area_name,
           (select count(*) from applications
                  where claims_status=1 and 
                    center_name=c.area_id) as cont
    FROM apparea c where cont<>0

我正在尝试从另一个表中获取字段和相关计数，但上述查询不起作用。该查询涉及两个不同的表(apparea、applications)。上面的查询有错误，我正在寻找替代方法来实现此目的。

Answer 1

列 cont 的别名在 WHERE 子句中不可用。您将需要使用与此类似的东西:

SELECT area_id, 
  area_name,
  cont
FROM
(
  SELECT area_id, 
    area_name,
    (select count(*) 
     from applications
     where claims_status=1 
     and center_name=c.area_id) as cont
  FROM apparea c
) c 
where cont<>0

这也可以使用LEFT JOIN来编写:

select c.area_id,
  c.area_name,
  a.cont
from apparea c
left join
(
  select count(*) cont,
    center_name
  from applications
  where claims_status=1 
  group by center_name
) a
  on c.area_id = a.center_name