In addition to knowing which tables are related to each other, you may need to know more about their type of relationship. This can help you decide whether and how you change the table structure, as well as help you troubleshoot if you run into problems.
Table relationships help bring the data together in a meaningful way. In the picture, for example, information about recordings and recording artists resides in different tables, yet through a table relationship you can easily determine which artists have made which recordings. And although information about tracks and recordings resides in different tables, you can determine which tracks appear on which recordings.
Table relationships are brought together by primary keys and foreign keys. A primary key uniquely identifies a record; it's often a set of numbers or characters or both, such as a driver's license number or customer identification number.
The primary key of one table becomes a foreign key in the other to which it is related. In the picture, for example, RecordingArtistID is the primary key in the Recording Artists table and a foreign key in the Recordings table. How do you know this? Because a field appears bold in the table in which it is the primary key.
In the music database, through the RecordingArtistID field, the Recording Artists table has a one-to-many relationship with the Recordings table because each artist can have multiple recordings.
And the Recordings table has a one-to-many relationship with the Tracks table through the RecordingID field because each recording has multiple tracks or songs.