Relational Databases
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Internal
Multiplicity (Cardinality)
The cardinality specifies how many instances of data can participate in a relationship. There are three different types of cardinality:
One-to-One (1:1) Relationships
In a one-to-one relationship, each constituent can have at most one relationship with the other constituent. 1:1 relationships are typically set up by a foreign key relationship in the database. The foreign key column can be set in either table. In practice, 1:1 relationships will be combined into a single table, rather than having a relationship between two separate data objects, so 1:1 relationships are not very common.
TODO
Object IDs
When persisting objects in a relational database, it is generally a good idea to have one field in the object that uniquely identifies the object.