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Referential integrity is a database concept. This type can garauntee relationships between tables these remain consistent. For this to work you need to have a primary and foriegn key.

A foreign key is a field in a relational table that matches a primary key column of another table. If cross referencing you can use the foreign key. Where as the primary key is unique for such as for some businesses such as stores ie : JD have card number for employees.. This is to identify them on their own and to be used to access past secuirity. As many other ways they can identify them for instance someone may try to access this. The use of a passcode to acess secuirity is a good way as someone may gain someone eleses card when they are not the owner of this.