Incercati asa:

CREATE PROC dbo.DeleteCust
@cust_num int = NULL
AS
IF EXISTS (SELECT cust_num FROM rental WHERE cust_num =
@cust_num)
BEGIN
RAISERROR(50001, 10, 1)
RETURN
END
DELETE FROM customer
WHERE cust_num = @cust_num
GO

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Cristian Andrei Lefter, SQL Server MVP
MCT, MCSA, MCDBA, MCAD, MCSD .NET,
MCTS, MCITP - Database Administrator SQL Server 2005
http://sqlserver.ro