Relationships
Even if relational databases aren’t called “relational” because the tables are related, one of their fundamental features is that the tables are related. Modelling relationships isn’t difficult: You add the primary key of one table to a related table (where it’s called a FOREIGN KEY) and then tell the database engine about the relationship. In SQL Server, you do this by setting a foreign key constraint, but different engines use different techniques. You can model three kinds of relationships, depending on the number of instances that can occur on either side.
One-to-One
One-to-one relationships are usually implemented for efficiency, but as we’ll see in Chapter 5 when we examine complex models, they can be used to imitate OOP inheritance ...
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