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

DECLARE @values TABLE (ProductId int)
INSERT @values (1)
INSERT @values (10)
INSERT @values (100)

SELECT CASE WHEN (SELECT COUNT(*) FROM @values v) = 
                 (SELECT COUNT(*) FROM Products p WHERE p.ProductId IN
                       (SELECT v.ProductId FROM @values v))
            THEN CAST(1 AS bit)
            ELSE CAST(0 AS bit)
       END [AreAllFound]