Consists of a set of constraints (logical schema objects) that define primary key and foreign key columns.
- Each small table must have a PRIMARY KEY constraint.
- The large table must contain columns that can be used to join the large table to smaller tables.
- Outer join queries produce the same results as the corresponding inner join query if there is a FOREIGN KEY constraint on the outer table. Note that the inner table of the outer join query must always have a PRIMARY KEY constraint on its join columns.