Third Normal Form

For a table to be in Third Normal Form, a non-key field cannot depend on another non-key field.

The table in Figure 6-5 violates Third Normal Form because the mgr_lname field depends on the mgr_emp_num field, which is not a key field.

Figure 6-5: A table that violates Third Normal Form

The solution is to split the Dept table into two tables, as shown in Figure 6-6. In this case, the Employees table, already stores this information, so removing the mgr_lname field from Dept brings the table into Third Normal Form.

Figure 6-6: Correcting Third Normal Form violations by creating two tables