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Relationships
Table is modelled as an entity
One or more columns in table are primary keys - the value stored in these columns uniquely identifies the row
Primary keys can be any data type - usually integers
Foreign keys - a column whose values correspond to values in the primary key of another table
When foreign key values of a row correspond to the primary ke values of a row in a different table we have a relationship between the rows
to-one relationship - one row corresponds to at most one row
to-many relationship - one row corresponds to many rows
To find the rows in another table related to the chosen row you must do a database join operation
Entity is represented as a class
Most variables of class contain simple data types
Some variables contain references to other EOs or to arrays of EOs
These variables represent to-one and to-many relationships
They correspond to foreign keys in the database tables
EOF performs the joins automatically to determine the references to objects to store in the variables
Reflective relationships
When the foreign key columns of a table contain values that appear in the primary key columns of the same table
The class will contain an instance variable that refers to objects (or an array of objects) of the same class
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Page updated September 7, 2004
