Assign Values of a column in SubQuery in SQL - sql

Question

I am trying to do following in SQL Server:
SELECT
PRODUCER_NAME, PRODUCER_ID,
(SELECT #X = #X + PRODUCT_NAME
FROM PRODUCT
WHERE PRODUCER_ID = PRODUCER.ID)
FROM
PRODUCER
There are two tables. Producer table is list of all producers. Product table stores product produced by producers. #x is varchar variable
Basically I want a list of all products, comma-separated by producer.
For example
Producer Products
-------- --------------------------
P1 ProductA,ProductB,ProductC
P2 ProductD,ProductE
I don't know if this is possible this way. Do anyone know how to do this without joining tables?

Answer 1

I don't have a way for you to assign multiple output comma-separated lists to a single varchar variable, but maybe you don't actually need that anyway. Try this:
SELECT Producer = PRODUCER.PRODUCER_NAME,
Products = STUFF(
(
SELECT N',' + PRODUCT.PRODUCT_NAME
FROM dbo.PRODUCT
WHERE PRODUCT.PRODUCER_ID = PRODUCER.ID
FOR XML PATH(''),
TYPE).value(N'./text()[1]', N'nvarchar(max)'),1,1,N'')
FROM dbo.PRODUCER;
On a large table, this kind of correlated subquery can be quite expensive. On SQL Server 2017+ we can use STRING_AGG() in a single pass:
SELECT Producer = PRODUCER.PRODUCER_NAME,
Products = STRING_AGG(PRODUCT.PRODUCT_NAME, N',')
FROM dbo.PRODUCT
INNER JOIN dbo.PRODUCER
ON PRODUCT.PRODUCER_ID = PRODUCER.ID
GROUP BY PRODUCER.PRODUCER_NAME;
Example db<>fiddle

Answer 2

If you want to concatenate names, you can do this:
select
P.PRODUCER_NAME, P.PRODUCER_ID,
stuff(
(
select ',' + T.PRODUCT_NAME
from PRODUCT as T
where T.PRODUCER_ID = P.PRODUCER_ID
for xml path(''), type
).value('.', 'nvarchar(max)')
, 1, 1, '') as PRODUCT_NAMES
from PRODUCER as P
Two notes:
Always use the table aliases for such a queries. To see why - delete alias name from query and drop PRODuCER_ID from PRODUCT table.
Use value method instead of implicit conversion to nvarchar to correctly work with names like 'Product & 1'.