Self join and inner join to remove duplicates - sql

Question

I am stuck on this and I am relatively new to SQL.
Here is the question we were given:
List the productname and vendorid for all products that we have
purchased from more than one vendor (Hint: you’ll need a Self-Join and
an additional INNER JOIN to solve, don't forget to remove any
duplicates!!)
Here is a screenshot of tables we are working with:
Here is what I have.....I know it is wrong. It works to a degree, just not exactly how the prof wants it.
SELECT DISTINCT productname, product_vendors.vendorid
FROM products INNER JOIN Product_Vendors
ON products.PRODUCTNUMBER = PRODUCT_VENDORS.PRODUCTNUMBER
INNER JOIN vendors ON Product_Vendors.VENDORID = vendors.VENDORID
ORDER BY products.PRODUCTNAME;
Expected output provided the prof:

Answer 1

I agree with #jarlh that additional information would be helpful- i.e. are there triplicates in the data or just duplicates, etc.
That said, this should get your started
SELECT
c.productname AS 'Product'
,a.vendorid AS 'Vendor1'
,b.vendorid AS 'Vendor2'
FROM
product_vendors AS a
JOIN
product_vendors AS b
ON
a.productnumber = b.productnumber
AND a.vendorid <> b.vendorid
JOIN
dbo.products AS c
ON
a.productnumber = c.productnumber
This will limit the population of 'Product Vendors' just to products with unmatching vendors.
From there you are joining to products to pull back product name.
Also- work on coding format, clean code makes the dream work :)

Answer 2

The solution to this problem is usually to count vendors per product with COUNT OVER and only stick with products with more than one. Simply:
select productname, vendorid
from
(
select
p.productname,
pv.vendorid,
count(*) over (partition by product) as cnt
from products p
join product_vendors pv using (productnumber)
)
where cnt > 1;
If this shall be done without window functions, then one option is to aggregate product_vendors and use this result:
select p.productname, pv.vendorid
from
(
select productid
from product_vendors
group by productname
having count(*) > 1
) px
join products p using (productid)
join product_vendors pv using (productid);
or check whether exists another vendor for the product:
select
p.productname,
pv.vendorid,
count(*) over (partition by product) as cnt
from products p
join product_vendors pv on pv.productnumber = p.productnumber
where exists
(
select *
from product_vendors other
where other.productnumber = pv.productnumber
and other.vendorid <> pv.vendorid
);
In neither of these approaches I see the need to eliminate duplicates, as there should be one row per product in products and one row per product and vendor in product_vendors. So I guess what your prof was thinking of is:
select distinct
p.productname,
pv.vendorid
from products p
join product_vendors pv on pv.productnumber = p.productnumber
join product_vendors other on other.productnumber = pv.productnumber
and other.vendorid <> pv.vendorid
This, however, is an approach I don't recommend. You'd combine all vendors for a product (e.g. with 10 vendors for one product you already have 45 combinations for that product only, if I'm not mistaken). So you'd create a large intermediate result only to dismiss most of it with DISTINCT later. Don't do that. Remember: SELECT DISTINCT is often an indicator for a poorly written query (i.e. unnecessary joins leading to too many combinations you are not actually interested in).

Answer 3

SELECT DISTINCT p.name AS product, v.id
FROM products p
INNER JOIN product_vendors pv ON p.id = pv.productid
INNER JOIN product_vendors pv2 ON pv.productid = pv2.productid AND pv.vendorid != pv2.vendorid
INNER JOIN vendors v ON v.id = pv.vendorid
ORDER BY p.name