Select 从两个表中使用左连接计数

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)

我只想用这个来计算总数buyers。问题是它似乎也在计算所有 assignments。

我试过按 b.id 和 ass.id_buyer 分组，我试过将 count(*) 更改为 count(b.id)。

没有任何效果。我该如何解决这个问题，让它只计算 buyers?

Answer 1

使用右外连接：

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)

Answer 2

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，这样它只会计算有任务的买家（否则为什么要加入 assignments？）。并使用 MAX(a.date_assigned) 以便它选择一个特定的分配日期，而不仅仅是来自任何分配的随机日期。

Select count with left join counting from both tables