find records grouped by two columns in mysql - mysql

Question

I want to find out all seller who have uploaded products in categories (electronics,clothing,furniture), so for 3 categories there can be 3 row against each seller . tables I have are
1.category{category_id,name},
2.seller {seller_id,username},
3.products{product_id,seller_id,category_id,title}
Note:There can be maximum 3 result (coz I'm searching in 3 categories) against one seller even if he added more than one product in single category
expected result:
**product_id** **category** **sellerUsername**
101 electronics kuldeep
211 furniture kuldeep
322 clothing kuldeep
167 electronics roman
245 furniture roman
247 clothing dangi
246 furniture dangi
..
..

Answer 1

if you need only the matching relation use inner join
select a.product_id, b.username, c.name
from products as a
inner join seller as b on b.seller_id = a.seller_id
inner join category as c on c.category_id = a.category_id
else use left join
select a.product_id, b.username, c.name
from products as a
left join seller as b on b.seller_id = a.seller_id
left join category as c on c.category_id = a.category_id

Answer 2

The general solution to your problem is to join the three tables together and then aggregate by seller and category. In my solution, I have arbitraily chosen the max product ID, in the absence of any logic for doing otherwise. The query is slightly tricky, in that we need to additionally join this result again to the category and seller tables to get the human readable category and seller names. The reason for this is the GROUP BY query should ideally be done by ID and not name, since conceivably two categories (or sellers) could have the same name but have different IDs.
SELECT t3.product_id,
COALESCE(t1.name, 'NA'),
COALESCE(t2.username, 'NA')
FROM
(
SELECT MAX(p.product_id) AS product_id,
c.category_id,
s.seller_id
FROM products p
LEFT JOIN category c
ON p.category_id = c.category_id
LEFT JOIN seller s
ON p.seller_id = s.seller_id
WHERE c.name IN ('electronics', 'clothing', 'furniture')
GROUP BY s.seller_id,
c.category_id
) t1
LEFT JOIN category t2
ON t1.category_id = t2.category_id
LEFT JOIN seller t3
ON t1.seller_id = t3.seller_id

Answer 3

Check Below Code.
SET #row_number:=0;
SET #db_names:= '';
SET #db_names2:= '';
select product_id,name as category ,username as sellerUsername
from (
select a.product_id, c.name ,b.username,
#row_number:=CASE WHEN #db_names=username and #db_names2=name THEN #row_number+1
ELSE 1 END AS row_number,#db_names:=username AS username2,#db_names2:=name AS name2
from products as a
left join seller as b on b.seller_id = a.seller_id
left join category as c on c.category_id = a.category_id
where name IN ('electronics', 'clothing', 'furniture')
)a where row_number < 2
order by sellerUsername,name;
Output :