You cannot enforce this with static RI (referential integrity) without the use of an artificial "helper" table, which is a bit of a mess to setup and maintian and not recommended. The recommended alternative is to use dynamic RI, (which almost always means triggers) to enforce this constraint.
[font="Times New Roman"]-- RBarryYoung[/font], [font="Times New Roman"] (302)375-0451[/font] blog: MovingSQL.com, Twitter: @RBarryYoung[font="Arial Black"]
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