6.2 Foreign Keys

	Foreign keys enable tables in a relational database to be linked together.
	A foreign key is a set of attributes in table T1 that reference another set of corresponding primary key attributes in table T2.
	The foreign key attributes in T1 must have the same domain as the corresponding primary key attributes in T2.
	Each value that occurs in the foreign key must also occur in the primary key unless the foreign key value is NULL.
	For example,

		If a library database has two tables: student member and department shown in Figure 6.2.
		The student member table has primary key matriculation number and the department table has primary key department number.
		The two tables can be linked with each other using the department number attribute.
		Thus, department number in the student member table will be a foreign key linked to the department number attribute in the department table which is its primary key.
		Note that a foreign key is usually denoted by an asterisk (*).

6.2 Foreign Keys