table to the DEPARTMENT table. This relationship is based on the department number field. Employee Flintstone works in department number 2, Gravel works in department 1. When a field in one table matches the primary key of another table, the field is referred to as a foreign key. A foreign key is a field or a group of fields in one table whose values match those of the primary key of another table. You can think of a foreign key as the primary key of a foreign table. In the personnel database example, the DEPT field in the EMPLOYEE table is a foreign key. The DEPT_NO field is still the primary key of the DEPARTMENT table. Lookup TableWhen a foreign key exists in a table, the foreign key's table is sometimes referred to as a lookup table. The DEPARTMENT table in our example is a lookup table for the EMPLOYEE table. The value of an employee's department can be looked up in the DEPARTMENT table. In Oracle, on-line screens or forms, have the ability to display a list of lookup table entries. The employee form could have a lookup list of departments from the DEPARTMENT table. The user simply highlights the preferred department, and Oracle automatically enters that department into the employee's record. This provides a means of ensuring that only valid data is entered in the database. A user can only select a listed department. Whether or not a lookup list is used in a form, Oracle will still check for a valid department when one is entered. However, if the form displays the list of possible departments, the user is less likely to enter an invalid department number. Not only does Oracle allow you to link multiple tables, it also maintains consistency between them. It can prevent you from deleting a department which still has employees in it. Or if you change an employee's ID number, then all records of their work hours will also reflect that change. Ensuring that the data among related tables is correctly matched is referred to as maintainin...
