Foreign keys

Foreign Keys

A Foreign Key allows you to link tables together. So if you have an Employee and need to assign an Asset to them you would need to include the EmployeeID in the Asset table, as shown in the Diagram following (Figure 1). Note that an asset can ONLY be assigned to a maximum of 1 employee.

It is common to use the same identifier in both tables (EmpID). To make the ERD clearer, the line between the entities is best drawn so that it connects the two attributes (EmpID) and if possible arrange the entities so that the line is horizontal (as shown).

When identifying which EmpID you are referring to you can use object notation e.g., tblEmployee.EmpID and tblAsset.EmpID.

Entity/Attributes notation showing keys

When we describe the entities and attributes we can also show the PKs and FK by underlining the PKs and putting a * next to the FKs.

• tblEmployee ( EmployeeID, Name, Address, Mobile, IRDNumber, Comments)

• tblClients ( ClientID, Name, Address, Mobile, GSTNumber, Comments)

• tblAssets (AssetID, ClientID*, Description, PurchaseDate, Value)

Foreign Keys in an ERD

To make the ERD clearer, the line between the entities is best drawn so that it connects the two attributes (EmpID) and if possible arrange the entities so that the line is horizontal (as shown). When identifying which EmpID you are referring to you can use object notation e.g., tblEmployee.EmpID and tblAsset.EmpID.