尝试基于另一个表获取结果 - WHERE 中不允许使用聚合函数

Question

我正在尝试编写一个查询，该查询将根据他们从另一个表中的捐赠总和选择所有用户，并按他们的捐赠总和排序。但我收到一个错误。我正在使用 postgres，但我想这同样适用于 mysql。

ERROR:  aggregate functions are not allowed in WHERE
LINE 8:  sum(donations.donation) = 400
         ^

用户表

id |   username   |
1     Larry Page
2     Sergey Brin

捐款表

id | user_id | donation |     date
1      1         100       2019-02-12
2      1         200       2019-02-13
3      2         500       2019-01-15
4      1         100       2019-04-10

这是我的查询

select
    users.username, sum(donations.donation) as donation_sum
from
    users
inner join donations
    on users.id = donations.user_id
where
    sum(donations.donation) = 400
and
    donations.date between '2019-01-01' and '2019-12-31'
order by
    donation_sum

我期待结果是这样的

username    |  donation_sum
Larry Page        400

Answer 1

要过滤聚合结果，您必须使用关键字HAVING：

select
    users.username, sum(donations.donation) as donation_sum
from
    users
inner join donations
    on users.id = donations.user_id
where
    donations.date between '2019-01-01' and '2019-12-31'
group by
    users.username
having
    sum(donations.donation) = 400
order by
    donation_sum