SQL Keys

                                                        

Types of SQL Keys

1.   Candidate Key

A Candidate Key is a set of one or more fields/columns that can identify a record uniquely in a table. There can be multiple Candidate Keys in one table. Each Candidate Key can work as Primary Key.

2.   Alternate key

A Alternate key is a key that can be work as a primary key. Basically it is a candidate key that currently is not primary key.

3.   Super Key

Super key is a set of one or more than one keys that can be used to identify a record uniquely in a table. Example: Primary key, Unique key, Alternate key are subset of Super Keys.

4.   Primary Key

Primary key is a set of one or more fields/columns of a table that uniquely identify a record in database table. It can not accept null, duplicate values. Only one Candidate Key can be Primary Key.

5.   Composite/Compound Key

Composite Key is a combination of more than one fields/columns of a table. It can be a Candidate key, Primary key.

6.   Unique Key

Unique key is a set of one or more fields/columns of a table that uniquely identify a record in database table. It is like Primary key but it can accept only one null value and it cannot have duplicate values.

7.   Foreign Key

Foreign Key is a field in database table that is Primary key in another table. It can accept multiple null, duplicate values.