4. The primary key of a relational table uniquely
identifies each record in the table.
It can either be a normal attribute that is
guaranteed to be unique (such as Roll No in a
student Table or Employee Number in Employee
Table)
5. A foreign key is a field in a relational table that
matches the primary key column of another
table.
The foreign key can be used to cross-reference
tables.
6. A relationship exists between two database
tables when one table has a foreign key that
references the primary key of another table.
7. One-to-one: Occur when there is exactly one
record in TableA that corresponds to exactly one
record in TableB.
One-to-many: Occur when each record in TableA
may have many linked records in TableB but each
record in TableB may have only one corresponding
record in TableA.
Many-to-many: Occur when each record in TableA
may have many linked records in TableB and vice-
versa.
