SQL: Return “true” if list of records exists?

Question

An alternative title might be: Check for existence of multiple rows?

Using a combination of SQL and C# I want a method to return true if all products in a list exist

Answer 1

Where is this list of products that you're trying to determine the existence of? If that list exists within another table you could do this

declare @are_equal bit
declare @products int

SELECT @products = 
     count(pl.id)
FROM ProductList pl
JOIN Products p
ON pl.productId = p.productId

select @are_equal = @products == select count(id) from ProductList

Edit:

Then do ALL the work in C#. Cache the actual list of products in your application somewhere, and do a LINQ query.

var compareProducts = new List(){p1,p2,p3,p4,p5};
var found = From p in GetAllProducts()
            Join cp in compareProducts on cp.Id equals p.Id
            select p;

return compareProducts.Count == found.Count;

This prevents constructing SQL queries by hand, and keeps all your application logic in the application.