Creating relationships

- I talked about Relationships in the earlier section…on Features of a Relational Database,…but it's time to get more specific.…You're organizing your database into individual tables,…and many of these tables will need to know…about each other.…If you've started by sketching out a rough…Entity Relationship Diagram, you begin by just attempting…to show that some kind of relationship exists…between your entities.…But to get closer to actually building this in a database,…we need to say what these relationships are.…

And while it is common in an ER Diagram to use a phrase…or a verb to describe it, like "contains" or "refers to"…or "belongs to," that's for your benefit.…The database can't describe it that way.…There are only three kinds of Relationship, of Cardinality,…between tables in a relational database,…and one of them dwarfs the others.…There's One-to-One, One-to-Many, and Many-to-Many.…Some people might add a fourth, of None at All.…

But the time to start defining these is yet another example…of how you can keep to a process, an Order,…