ORDER BY VS GROUP BY COUNT（*）

Question

问题： 为什么我无法在

GROUP BY num_accounts 中

select c.cust_id, COUNT(*) num_accounts
from customer c LEFT JOIN
account a on a.account_id = c.cust_id
GROUP BY c.cust_id, num_accounts;

相反，我必须将' num_accounts '放入 ORDER BY 子句：

select c.cust_id, COUNT(*) num_accounts
from customer c LEFT JOIN
account a on a.account_id = c.cust_id
GROUP BY c.cust_id
ORDER BY num_accounts;

？

我的假设是 num_accounts 是由聚合生成的列，应使用GROUP BY。

谢谢

Answer 1

此SQL的处理顺序与写入顺序不同。首先加入，在哪里，分组，选择，然后最后排序。这意味着分阶段不存在num_accounts。您可以改用它

GROUP BY c.cust_id, COUNT(*)