FIX: A MERGE statement may not enforce a foreign key constraint when the statement updates a unique key column that is not part of a clustering key and there is a single row as the update source in SQL Server 2008

use tempdb;

drop table sale, product;
create table product(pno int not null primary key, name char(30), pAlternateKey char(6) not null unique);
create table sale(sno int not null primary key, pAlternateKey char(6) not null references product(pAlternateKey));
insert product values(1, 'Office Chair', 'ochair');
insert sale values(1, 'ochair')

-- No violation of foreign key constraint is detected. However, one should be.
merge into product
using (select 'Office Chair2' as name, 1 as pno, 'oxx' as pAlternateKey) as src
on product.pno = src.pno
when matched then
   update set product.pAlternateKey = src.pAlternateKey, 
              product.name = src.name
when not matched then
   insert values(src.pno, src.name, src.pAlternateKey);

insert product values(1, 'Office Chair', 'ochair');
insert sale values(1, 'ochair')
-- A foreign key constraint violation is detected, and the update fails.
declare @source table 
   (name nchar(30), pno int, pAlternateKey nchar(30));
insert into @source values('Office Chair2',1,'oxx');

merge into product
using @source as src
on product.pno = src.pno
when matched then
   update set product.pAlternateKey = src.pAlternateKey, 
              product.name = src.name
when not matched then
   insert values(src.pno, src.name, src.pAlternateKey);