Who sell with lowest price - mysql

Question

I have a database with tables:
suppliers:
fid , name
1 | 'Andrey'
2 | 'lucas'
products:
pid , name
1 | 'X'
2 | 'Y'
prdtFrn:
pid , fid , price
---------------- supplier 'andrey'
1 | 1 | 19.00
2 | 1 | 16.00
----------------- supplier 'lucas'
1 | 2 | 14.00
2 | 2 | 18.00
Querying the products, I get all products that have registred, but I need know who sells current product of 'product table' as lowest price, I don't know how to write the query

Answer 1

select p.*
from (
select pid, min(price) as MinPrice
from prdtFrn
group by pid
) pm
inner join prdtFrn p on pm.pid = p.pid and pm.MinPrice = p.price
You will see multiple rows per product where two suppliers are selling a product at the same price.

Answer 2

As with any complex SQL, build it up, step-by-step.
SELECT s.name AS s_name, p.name AS p_name, sp.price
FROM suppliers AS s
JOIN prdtFrn AS sp ON s.fid = sp.fid
JOIN products AS p ON p.pid = sp.pid
This gives you the list of seller, product and price. Now, presumably, for each product, you want the details of the supplier name who sells the product for the smallest price:
SELECT p_name, MIN(price) AS min_price
FROM (SELECT s.name AS s_name, p.name AS p_name, sp.price
FROM suppliers AS s
JOIN prdtFrn AS sp ON s.fid = sp.fid
JOIN products AS p ON p.pid = sp.pid
) AS spp
This gives you the minimum price for each product. Now you need to join that result with the first query, to yield the answer:
SELECT spp.p_name, spp.s_name, min_price
FROM (SELECT s.name AS s_name, p.name AS p_name, sp.price
FROM suppliers AS s
JOIN prdtFrn AS sp ON s.fid = sp.fid
JOIN products AS p ON p.pid = sp.pid
) AS spp
JOIN (SELECT p_name, MIN(price) AS min_price
FROM (SELECT s.name AS s_name, p.name AS p_name, sp.price
FROM suppliers AS s
JOIN prdtFrn AS sp ON s.fid = sp.fid
JOIN products AS p ON p.pid = sp.pid
) AS spp
) AS mp
ON mp.p_name = spp.p_name AND mp.min_price = spp.price;
Then, when it's working, you can optimize...for example, the minimum price calculation doesn't need the supplier information:
SELECT spp.p_name, spp.s_name, mp.min_price
FROM suppliers AS s
JOIN prdtFrn AS sp ON s.fid = sp.fid
JOIN (SELECT pid, MIN(price) AS min_price
FROM prdtFrn
GROUP BY pid
) AS mp
ON mp.pid = sp.pid AND mp.min_price = sp.price;
My first query went awry because I went for more information than was needed in the very first query; this is much simpler. So, the lessons here are:
Build up the query piece-by-piece.
Don't be satisfied with the first draft.