從兩個表中選擇帶有左聯接計數的計數

Question

我有以下查詢：

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)

我只想以此來計算buyers總數。 問題在於它似乎也在計算所有assignments 。

我試過按b.id和ass.id_buyer分組，我試過將count(*)更改為count(b.id) 。

沒用。 我該如何解決，這樣才算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

我也從改變LEFT JOIN到INNER JOIN ，這樣只會算買家誰擁有分配（否則為什么有參加assignments ？）。 並使用MAX(a.date_assigned)以便它選擇特定的分配日期，而不僅僅是從任何分配中選擇隨機日期。

Answer 2

使用權外加入：

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)