SQL Keys

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

 

Types of SQL Keys

We have following types of keys in SQL which are used to fetch records from tables and to make relationship among tables or views.

  1. 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.

  2. 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.

     

  3. 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.

  4. 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.

  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

    Uniquekey 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.

  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.

    Example : We can have a DeptID column in the Employee table which is pointing to DeptID column in a department table where it a primary key.

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