从两个表中选择带有左联接计数的计数

Question

I have the following query:

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)

I only want this to count the total buyers . The problem is it appears to be counting all of the assignments as well.

I've tried grouping by b.id and ass.id_buyer , I've tried changing count(*) to count(b.id) .

Nothing works. How do I fix this so it only counts the buyers ?

Answer 1

Use 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

I've also changed from LEFT JOIN to INNER JOIN so that it will only count buyers who have assignments (otherwise why join with assignments ?). And used MAX(a.date_assigned) so that it selects a particular assignment date, not just a random date from any of the assignments.

Answer 2

使用权外加入：

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)