从两个表中选择左连接计数的计数

Question

我有以下查询：

select count(*) as `count`, ass.date_assigned as date_assigned
from `buyers` as b
left join `assignments` as ass on (ass.id_buyer = b.id)

我只想要计算总数buyers。问题是它似乎也在计算所有assignments。

我已尝试按b.id和ass.id_buyer分组，我们已尝试将count(*)更改为count(b.id)。

没有任何作用。如何解决此问题，以便仅计算buyers？

Answer 1

使用COUNT(DISTINCT)：

SELECT COUNT(DISTINCT b.id) AS count, MAX(a.date_assigned) AS last_date_assigned
FROM buyers AS b
INNER JOIN assignments AS a ON a.id_buyer = b.id

我也从LEFT JOIN更改为INNER JOIN，因此它只会计算有作业的买家（否则为什么要加入assignments？）。并使用MAX(a.date_assigned)以便选择特定的分配日期，而不仅仅是来自任何分配的随机日期。

Answer 2

使用正确的外部加入：

select count(*) as `count`, ass.date_assigned as date_assigned from `buyers` as b right outer join `assignments` as ass on (ass.id_buyer = b.id)