从一个表中获取行数，其中另一个表中的至少一个相关项与条件匹配

Question

I have 'products' table and related 'variations' table, one product can have one or more variations. 'variations' table has 'status' column, its value can be 0 or 1. I want to get the number of products (COUNT()) which have at least one variation of status 1. How to make a query that would do that? [EDIT] Ok, I thought that if I simplify the question I will get away with the table structure, but, here we go (only columns relevant to the question and some mock data):

It's actually 3 linked tables: table 1: 'products'

id	name
1	t-shirt
2	shoes
3	shorts

table 2: variations

id	product_id
1	1
2	1
3	2
4	2
5	3
6	3
7	3

table 3: stock

variation_id	quantity	status [0 or 1]
1	10	1
2	15	1
3	0	0
4	0	0
5	0	0
6	3	1
7	0	0

So, with this data, I want to know how many products there are that have at least 1 of its 'variations' of 'status' 1 - in this example it would be 2 (product 1 and 3 have some variations with status 1, product 2 does not).

Answer 1

You just need SUM all the quantity GROUP BY products.id with criteria is stock.status equal 1.

SELECT id, name, SUM(quantity) AS total_quantity
FROM Products pr
LEFT JOIN Variations va ON pr.id = va.product_id
LEFT JOIN Stock st ON st.variation_id = va.id
WHERE st.status = 1
GROUP BY pr.id

Answer 2

Join two tables and apply where filter on status column

select count(*) as cnt
from
products p
join variations v
on p.product_id = v.product_id
where status = 1