One of the databases that I’m working on has some quirky behavior that I want to account for in the entity-relationship diagram.
One of the behaviors is that there is a ‘booking’ table and a ‘invoice’ table. When a ‘booking’ is invoiced, then the record is inserted into the ‘invoice’ table and then deleted from the ‘booking’ table.
However, a reference is still kept of the booking number.
How do we model this? Big arrow between the tables and some text beside it describing what happens?
No, changing the database schema is not possible at this point in time
Edit: This is the type of diagram that I want to use:
alt text http://img813.imageshack.us/img813/5601/erdartistperformssong.png
Link
If, by ERD, you mean the original “Chen” diagrams where the relationship was words written in a diamond, then you have a relationship between between Booking and Invoice. It’s a special kind of relationship that’s NOT implemented with a simple foreign key; it’s implemented via a complicated move and a constraint.
If, by ERD, you mean the diagrams that ERwin draws, then you don’t have an easy way to do this. It tends to focus you on drawing PK-FK relationships. You have a non-PK-FK relationship between these things. Some kind of line with text is about all you can do.
Arrows, BTW, aren’t appropriate because the ERD shows the “state” of the database. Data flowing around isn’t part of an ERD. You do have a relationship, it’s just not a typical PK-FK relationship. It’s an atypical relationship based on rows existing in some places and not existing in others.
In the UML you can easily draw this as a “constraint” among the relationships.