连接两个表和两个列

Question

I have articles and discounts table. Articles could have family and subfamily (or not, could be blank).

Articles:

id | name | price | family | subfamily
1  | art1 | 5     | F01    | A02
2  | art2 | 5.5   | F02    |

Discounts

id | customer_id | family | subfamily | discount
1  | ABC123      | F01    | A02       | 40%
2  | CBD321      | F02    |           | 30%

I need to retrieve articles with their discount based on: customer_id (customer who will do order) family and subfamily. Articles could have discount based on his family only (subfamily is blank) or on his family and subfamily but both need to be associed to customers id on the table. If one articles doesnt match anything, his discount would be null for that customer.

How can I do this? I tried this, but only can retrieve rows as if it were INNER JOIN

SELECT
  a.*,
  d.discount
FROM articles a
LEFT JOIN discounts AS d
  ON a.family = d.family
  AND a.subfamily = d.subfamily
WHERE d.customer_id = 'ABC123'

Answer 1

Move the discounts condition from WHERE to ON to get true LEFT JOIN result:

SELECT a.*, d.discount
FROM articles a
LEFT JOIN discounts AS d ON a.family = d.family AND a.subfamily = d.subfamily
  AND d.customer_id = 'ABC123'