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foreign key

One or more table fields (columns) that refer to the primary key field or fields in another table. A foreign key indicates how the tables are related  the data in the foreign key and primary key fields must match, though the field names do not have to be the same. For example, a Products table might contain the foreign key SupplierNumber, which relates to the primary key SupplierID in a Suppliers table.

A foreign key can be Null; if a foreign key consists of more than one field and any of those fields is Null, all the fields must be Null.