Database Basics - Foreign Keys |
A foreign key is a relationship or link between two tables which ensures that the data stored in a database is consistent.
The foreign key link is set up by matching columns in one table (the child) to the primary key columns in another table (the parent).
In the example below, there is a link between the Company and Contact tables. The Company table is the parent table in the link. The Contact table is the child: the Company_ID field in the Contact table indicates which Company a Contact belongs to.
The multiplicity of a foreign key defines how many records in the child table can link to a record in the parent table. In the example above the child multiplicity is "many" since one company can have any number of contacts.
Each child record in a foreign key relationship must have a matching parent record. A user of your database will not be allowed to delete a Company that has Contacts linked to it unless the foreign key is set up to do a cascade delete. The cascade delete option means that whenever a record in the parent table is deleted, every matching record in the child table is also deleted.
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