Define candidate key, alternate key, composite key.
Candidate key: A column or a set of columns can be called as candidate key if they identify each row of a table uniquely. A table can have multiple candidate keys. One of them is specified as Primary key and rest of them can be called as alternate key.
Alternate key: There can be more than one keys which can identify each row of the table uniquely. One of them is defined as primary key and rest of them is called alternate keys of the table.
Composite Key:A key formed by combining at least two or more columns.