A column or set of columns in a table whose values are required to match at least one PrimaryKey values of a row of another table, or (in some interpretations of the term) be a distinguished ''null'' value.

It should be noted that the ForeignKey concept is not part of the fundamental RelationalModel, but is only a special (and very common) case of a DatabaseConstraint involving two tables. Database management products often provide explicit, convenient support for this type of constraint.

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RelationalWeenie''''''s don't like to hear this (it is often claimed that RelationalHasNoPointers), but the ForeignKey is the relational database equivalent of an object reference.  While operating at a higher level then a physical address (ForeignKey''''''s say nothing about the underlying physical representation of the database), they serve the same purpose--''to allow data in one entity to reference another''.  Note the word reference, not pointer.



CategoryDatabase