mysql - 如何使用外表编写子查询

Question

我需要编写一个 MySQL 查询来获取每个月的帐户数量、新帐户数量和旧帐户数量。 旧帐户的条件描述为 cond1。

我已经尝试过:

select month(a.ct) as month,
count(select id from a where cond1) as oldAccount,
count(select id from a where !cond1) as newAccount
from accounts a
where ~conditions~
group by month(a.ct)

它并不完全有效。

一般来说，我需要知道如何在子查询中使用外表。 谁能帮助我吗？

Answer 1

通常，count() 会进入子查询内部:

select month(a.ct) as month,
      (select count(id) from a where cond1) as oldAccount,
      (select count(id) from a where !cond1) as newAccount
from accounts a
where ~conditions~
group by month(a.ct);

但是，外部查询中有一个聚合，因此这有点复杂。您的关联条件大概是在 ID 上而不是在月份上。这种不一致会影响上面的查询。

最好的方法可能是将其表达为join，但这可能取决于条件。否则，您可以使用子查询:

select month(a.ct) as month,
       sum(oldAccount), sum(newAccount)
from (select a.*,
             (select count(id) from a where cond1) as oldAccount,
             (select count(id) from a where !cond1) as newAccount
      from accounts a
      where ~conditions~
     ) t
group by month(a.ct);

编辑:

如果通过 a，您想要外部 accounts 表，则只需使用条件聚合:

select month(a.ct) as month,
      sum(cond1) as oldAccount,
      sum(!cond1) as newAccount
from accounts a
where ~conditions~
group by month(a.ct);