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    <p>No, a foreign key constraint always references exactly one parent table.</p> <p>This question comes up frequently. Here are some of my past answers to it:</p> <ul> <li><a href="https://stackoverflow.com/questions/922184/why-can-you-not-have-a-foreign-key-in-a-polymorphic-association/922341#922341">Why can you not have a foreign key in a polymorphic association? </a> </li> <li><a href="https://stackoverflow.com/questions/441001/possible-to-do-a-mysql-foreign-key-to-one-of-two-possible-tables/441111#441111">Possible to do a MySQL foreign key to one of two possible tables?</a> </li> <li><a href="https://stackoverflow.com/questions/759008/referencing-foreign-keys-in-the-same-column/759100#759100">Referencing foreign keys in the same column</a> </li> <li><a href="https://stackoverflow.com/questions/987654/in-a-stackoverflow-clone-what-relationship-should-a-comments-table-have-to-quest/987709#987709">In a StackOverflow clone, what relationship should a Comments table have to Questions and Answers?</a> </li> <li><a href="https://stackoverflow.com/questions/2002985/mysql-conditional-foreign-key-constraints/2003042#2003042">MySQL - Conditional Foreign Key Constraints</a> </li> <li><a href="https://stackoverflow.com/questions/2098429/how-to-handle-an-or-relationship-in-an-erd-table-design/2098477#2098477">How to handle an “OR” relationship in an ERD (table) design?</a> </li> <li><a href="https://stackoverflow.com/questions/3145084/mysql-two-n1-relations-but-not-both-at-once/3145711#3145711">MySQL: Two n:1 relations, but not both at once</a> </li> </ul> <p>For more on Polymorphic Associations, see my presentation <a href="http://www.slideshare.net/billkarwin/practical-object-oriented-models-in-sql" rel="nofollow noreferrer">Practical Object-Oriented Models in SQL</a> or my book, <a href="http://www.pragprog.com/titles/bksqla/sql-antipatterns" rel="nofollow noreferrer">SQL Antipatterns: Avoiding the Pitfalls of Database Programming</a>.</p>
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    1. COwow ... how did u manage to find all those links +1!
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    2. COsearch by user id and by tag: `user:20860 [polymorphic-associations]`
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