CREATE TABLE GeneralInfo (EntityType INT, EntityId INT, CONSTRAINT PK_GeneralInfo PRIMARY KEY (EntityType, EntityId))
CREATE TABLE Customers (Id INT, Name VARCHAR(50), CONSTRAINT PK_Customers PRIMARY KEY (Id))
CREATE TABLE Suppliers (Id INT, Name VARCHAR(50), CONSTRAINT PK_Suppliers PRIMARY KEY (Id))
GO

CREATE FUNCTION CheckEntityExists(@EntityType INT, @EntityId INT) RETURNS INT AS
BEGIN
	DECLARE @ret INT = 0
	SELECT @ret = COUNT(*) FROM (SELECT 1 Record FROM Customers WHERE @EntityType = 0 AND Id = @EntityId UNION ALL SELECT 1 FROM Suppliers WHERE @EntityType = 1 AND Id = @EntityId) t
	RETURN @ret
END
GO

ALTER TABLE GeneralInfo ADD CONSTRAINT CheckEntityExistsConstraint CHECK (dbo.CheckEntityExists(EntityType, EntityId) > 0)
GO

INSERT INTO GeneralInfo (EntityType, EntityId) VALUES (0, 1)
GO
--ERROR:
--Msg 547, Level 16, State 0, Line 1
--The INSERT statement conflicted with the CHECK constraint "CheckEntityExistsConstraint". The conflict occurred in database "DBA_Teste", table "dbo.GeneralInfo".
INSERT INTO Customers (Id, Name) VALUES (1, 'Customer 1')
INSERT INTO GeneralInfo (EntityType, EntityId) VALUES (0, 1)
-- OK.
DELETE FROM Customers
-- OK. Should not be allowed to delete since the record is "referenced"..