Your company would like to make a song player appl. Your requirements include: A song has a unique identifier (lets call this SONG ID) You would like to support multiple playlists. Each playlist has a unique identifier (you can decide whether this is the playlist name or a generate identifier). Your model needs to support having a song in multiple playlists, and a playlist having multiple songs. Thus, PLAYLIST:SONG is M:N and you need another entity (i.e. something to bridge the many to many). You and your coworkers are discussing the design of the bridging entity. Some alternative designs include: Having a composite PK made up of the FKs from the PLAYLIST and SONG entities. Having an generated PK for the bridge Show (using ERDs) and discuss pros and cons of these two approaches . If a requirement was that a song should be limited to once in a given playlist, how does that influence your design? If you wanted to allow a song to be in a given playlist numerous times, how does that influence your design? If you wanted to give ordering to a playlist (e.g. play the songs in a user-defined order), how does that influence your design?