Display Highest Offer, But Still Display With No Offer - php

Question

I'm trying to display businesses along with their highest discount offer. But I still would like to display businesses with no offer.
Businesses are stored in business_tb
business_id | business_name
------------+---------------
1 | aaa
2 | bbb
3 | ccc
offered discounts by those businesses are stored in deal_offer_tb
deal_offer_id | business_id | deal_id
--------------+-------------+----------
1 | 1 | 3
2 | 1 | 2
3 | 2 | 0
4 | 1 | 1
5 | 3 | 3
and types of discounts are stored in deal_tb.
deal_id | discount
--------+----------
1 | 40%
2 | 30%
3 | 20%
4 | 10%
So the display I wanted should look something like this:
1 | aaa | 40%
2 | bbb | ---
3 | ccc | 20%
But with my current query:
SELECT a.business_id, a.business_name, c.discount
FROM business_tb a
LEFT JOIN (SELECT min(deal_id) AS deal_id, business_id FROM deal_offer_tb GROUP BY business_id) b ON a.business_id = b.business_id
LEFT JOIN deal_tb c ON b.deal_id = c.deal_id
I only get:
1 | aaa | 40%
3 | ccc | 20%
It does not display businesses with no offered discounts.
How am I suppose to get my desired output?
UPDATE: I don't know what happened earlier, but my query is working the way I wanted it. Thanks to the effort of those who answered. Appreciate it, big time!

Answer 1

I would approach this by using a subquery to find the greatest discount for each business, joining the deal_offer_tb and deal_tb tables. Then, join this subquery to the business_tb table to get the final result. Note that I use an initial LEFT JOIN to account for that a given business may not even have an deals associated with it. In that case, I assign a maximum discount of 0 to that business (which makes sense, since then the full regular price would apply).
SELECT
t1.business_id,
t1.business_name,
COALESECE(t2.max_discount, 0) AS max_discount
FROM business_tb t1
LEFT JOIN
(
SELECT t1.business_id, MAX(t2.discount) AS max_discount
FROM deal_offer_tb t1
INNER JOIN deal_tb t2
ON t1.deal_id = t2.deal_id
GROUP BY t1.business_id
) t2
ON t1.business_id = t2.business_id

Answer 2

This query is essentially your query (with table aliases):
SELECT b.business_id, b.business_name, d.discount
FROM business_tb b LEFT JOIN
(SELECT MIN(deal_id) AS deal_id, business_id
FROM deal_offer_tb dot
GROUP BY business_id
) dot
ON b.business_id = dot.business_id LEFT JOIN
deal_tb d
ON d.deal_id = dot.deal_id;
By the definition of LEFT JOIN, it will keep all rows in business_tb, regardless of whether or not there are matches in the rest of the FROM clause. You have no additional filtering (via WHERE) or aggregation. Hence, this should returns all the rows in business_tb.

Answer 3

Below is one way to do the query:
SELECT
a.business_id, a.business_name, b.max
FROM business_tb a
INNER JOIN (
SELECT
c.business_id, MAX(d.discount) AS max
FROM deal_offer_tb c
LEFT JOIN deal_tb d ON c.deal_id = d.deal_id
GROUP BY c.business_id
) b ON a.business_id = b.business_id;
You don't have deal_id 0 in deal_tb, so I assume it's null for deal_offer_id 3.