另一个联接表上的SQL联接约束列

Question

So practically I have 3 tables:

products
id

orders
id
status

order_items
id
order_id
product_id
quantity

So I want to query by products table as the main one. I want to know how many each one has been ordered, hence

SELECT products.*, SUM(order_items.quantity) AS sale_count 
 FROM products 
 LEFT JOIN order_items ON products.id = order_items.product_id 
 GROUP BY products.id

Everything works fine, no problem. But this query will count any order_items for each row, now I want to count only if the order status is done . But the problem is to get the status of that order_items you need to reference orders right?

 SELECT products.*, SUM(order_items.quantity) AS sale_count 
 FROM products 
 LEFT JOIN order_items ON products.id = order_items.product_id 
 LEFT JOIN orders ON order_items.order_id = orders.id AND orders.status = 'done' 
 GROUP BY products.id

But this doesn't make a constraint on the order_items instead only order which has the status of done will be shown in the table else NULL.

How to approach this problem? Thank you for your kind help. I really appreciate it!!

Answer 1

I think you just want conditional aggregation:

SELECT p.*,
       SUM(CASE WHEN o.status = 'done' THEN order_items.quantity END) AS sale_count
FROM products p LEFT JOIN
     order_items oi
     ON p.id = oi.product_id LEFT JOIN
     orders o
     ON oi.order_id = o.id 
GROUP BY p.id;

I moved the 'done' condition to a case statement. You can leave it in the on clause as well, if you like. I just think this logic is more explicit about your intention.

Answer 2

It's not 100% clear to me what you are asking, but I think you are asking how to count orders for which the order status is done without throwing out results for which the order status is not done. If so, then the following should work:

    SELECT products.*, 
    SUM(CASE WHEN orders.status = 'done' 
        THEN order_items.quantity 
        ELSE 0 END) AS sale_count 
    FROM products 
    LEFT JOIN order_items 
        ON products.id = order_items.product_id 
    LEFT JOIN orders 
        ON order_items.order_id = orders.id  
    GROUP BY products.id