SQL Server query troubles, many-many relationship - sql

Question

Not sure how to word this question in one line, apologies for the title...
I have 3 tables in my database, for example:
Shop
Item
ShopStock
Shop and Item have a many-many relationship and so the ShopStock table links them.
The fields in ShopStock are:
ID
ShopID
ItemID
CurrentStock
I want to list the items, showing how much stock each shop has, but I'm having trouble with the SQL. Something like this:
ITEM TESCO STOCK ASDA STOCK SAINSBURY STOCK
Apples 5 20 74
Pears 1000 32 250
How do I build the SQL query to display the data like this?

Answer 1

This would be easier to list as item,shop,currentstock in multiple rows. As is, unless you know the number of shops, you're going to need to use dynamic sql for this. If you know the number of potential shops, you can use PIVOT to return your results.
Something like this assuming you had 2 shops (shop1 and shop2):
select item_name, [Shop1], [Shop2]
from
(
select item_name, shop_name, currentstock
from item i
join shopstock ss on i.item_id = ss.item_id
join shop s on s.shop_id = ss.shop_id
) x
pivot
(
max(currentstock)
for shop_name in ([Shop1],[Shop2])
) p
SQL Fiddle Demo
Here is the dynamic sql approach as I suspect you don't know the number of possible shops:
DECLARE #cols AS NVARCHAR(MAX),
#query AS NVARCHAR(MAX)
select #cols = stuff((select distinct ',' + quotename(shop_name)
from shop
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)')
,1,1,'')
set #query = 'select item_name,' + #cols + '
from
(
select item_name, shop_name, currentstock
from item i
join shopstock ss on i.item_id = ss.item_id
join shop s on s.shop_id = ss.shop_id
) x
pivot
(
max(currentstock)
for shop_name in (' + #cols + ')
) p '
execute(#query)
SQL Fiddle Demo

Answer 2

You might need to add JOINS to get specific names, but this is the idea you're after:
SELECT ItemID
, MAX(CASE WHEN ShopID = 'Tesco' THEN CurrentStock ELSE 0 END)'Tesco Stock'
, MAX(CASE WHEN ShopID = 'ASDA' THEN CurrentStock ELSE 0 END)'ASDA Stock'
, MAX(CASE WHEN ShopID = 'Sainsbury' THEN CurrentStock ELSE 0 END)'SainsburyStock'
FROM ShopStock
GROUP BY ItemID
Assuming one entry per item per shopID. If there are multiples then you would have to SUM() them, but the idea is the same.