Different Types of SQL Keys | Learn SQL

Types of SQL Keys

A key is a single or combination of multiple fields in a table. Its is used to retrieve records or rows from data table according to the condition/requirement. Keys are also used to create relationship among different database tables or views. Below are different SQL Keys.

  • Super Key
  • Candidate Key
  • Primary Key
  • Alternate key
  • Composite/Compound Key
  • Unique Key
  • Foreign Key

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.

Candidate Key


A Candidate Key is a set of one or more 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.

Example: In a student table, ID, RollNo and EnrollNo are Candidate Keys since all these three fields can be work as Primary Key.

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.

Alternate key


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

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.

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 can not have duplicate values.

Foreign Key


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

Defining SQL Keys

For More CREATE Table In SQL

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