3.3. Foreign Key
Foreign key constraints enforce referential integrity. A foreign key constraint restricts the domain of a column value. An example is to restrict a STATE abbreviation to a limited set of values in another control structure—that being a parent table.
The term “lookup” is often used when referring to tables that provide this type of reference information. In some applications, these tables are created with this keyword—a practice we'll use here with the example STATE_LOOKUP.
Start with creating a lookup table that provides a complete list of state abbreviations. Then use referential integrity to ensure that students have valid state abbreviations. The first table is the state lookup table with STATE as the primary key.