☞ A key allow us to identify an attribute or a set of attributes on the basis of which a table is identified.
☞ They are used to establish and identify relation among two or more tables.
☞ They also ensure that each record within a table can be uniquely identified by combination of one or more fields within a table.
☞ The different types of keys in an RDBMS are as follows :
1. Primary Key 2. Candidate Key 3. Alternate Key 4. Foreign Key 5. Composite Key
☞ It uniquely identifies a tuple/record in a table.
☞ It cannot be left null.
☞ One table can have only one primary key.
☞ All those attributes/fields that are capable of becoming a primary key are known as candidate key.
☞ A candidate key that is not the primary key is called an alternate key.
☞ Alternate key = Candidate keys - Primary key
☞ It is a non-key attribute whose value is derived from the primary key of another table.
☞ If in a table we use more than one field to identify records, it is known as composite key.