In our earlier illustration, we saw that a foreign key field created in a table acts as an ambassador. To make this effective, a formal relationship must be created between that field and the primary key field of the table it represents. There should be some type of link between both fields.
When you are creating a Lookup field, you are indicating that the value entered in this particular field will come from another table, and you specify the originating table. The originating table is the parent table. The target table is the child table.
The reason you established Primary Keys in your tables is because these are the fields used to build relationships between tables. They are used to verify the uniqueness of data.
They also, prevent data in relationships being mixed. You can build a reliable relationship only between data of the same kind. We also saw that, to make it easy to recognize the primary key of a parent table and the foreign key of a child table, both fields should have the same name, although this has little to do with the database itself: as long as both fields have the same data type, the relationship can be created.
Tables and fields relationships are created and managed in a special window called the Relationship window. To display it, on the Database toolbar, you can click the Relationships button. Alternatively, on the main menu, you can click Tools -> Relationships. After clicking one of those, if no relationship exists among the tables in the current database, the Show Table dialog box would come up, asking you to select the tables whose relationship(s) you want to create. If at least one relationship has been created between two tables, the Relationship window would come up and display that relationship or the already existing relationships. In this case also, if a table of the current database is not represented in the window, to add it, display the Show Table dialog box, select the table, click Add and click Close. You can create a relationship only between two tables that are present on the Relationships window. This means that even if a table is part of your database and you want to link it to another table (of your database), if the table has not been added to the Relationships window, you cannot create or manage its relationship to another table. Of course, there are other ways you can create relationships without using the Relationships window but the Relationships window gives you advanced and detailed means of creating and managing relationships.
When the Relationships window is displaying, a new menu group is added to the main menu: it is the Relationships. To establish a relationship that does not yet exist between two tables, you can drag the primary key from the parent table to the foreign key of the desired table. If
you drag it accurately, the relationship would be acknowledged and you can just click Create to make it formal. If you dropped the primary key on the wrong field, you would have time to select the appropriate fields in the Edit Relationship dialog box.
After working with the Relationships window, you can close it. You would be asked to save it in order to keep the relationship(s) created.