How To Use PRIMARY KEY Constraint In SQL

In this article, we will learn how to use the Primary Key constraint in SQL.

The PRIMARY KEY constraint is used to identify each record uniquely in a table.

Primary keys cannot contain NULL values and must contain UNIQUE values.

A table can have only one PRIMARY KEY but this primary key can be consist of single or multiple columns.

PRIMARY KEY – CREATE TABLE: 

Please check How To Create Table Using Query In SQL, to get a brief description of the CREATE TABLE statement.

Example-1

The subsequent statement would create a PRIMARY KEY constraint on the “ID” column when the “Article” table is created:

CREATE TABLE Article (
    ID int PRIMARY KEY,
    Title varchar(100),
    Author varchar(100),
    Views int 
);

Example-2

The subsequent statement would create a PRIMARY KEY constraint, which is made up of two columns (ID + Title) when the “Article” table is created:

CREATE TABLE Article (
    ID int,
    Title varchar(100),
    Author varchar(100),
    Views int,
    CONSTRAINT PKArticle PRIMARY KEY (ID, Title)
);

PRIMARY KEY – ALTER TABLE: 

Please check How To Alter Table Using Query In SQL, to get a brief description of the ALTER TABLE statement.

• To ADD a PRIMARY KEY Constraint

Syntax

ALTER TABLE TABLE_NAME 
ADD CONSTRAINT CONSTRAINT_NAME PRIMARY KEY (column1, column2, ...columnN);

Example

The subsequent statement would add a PRIMARY KEY constraint on the “ID” column using ALTER TABLE statement:

ALTER TABLE Article 
ADD CONSTRAINT PKArticle PRIMARY KEY (ID);

• To DROP a PRIMARY KEY Constraint

Syntax

ALTER TABLE TABLE_NAME 
DROP CONSTRAINT CONSTRAINT_NAME;

Example

The subsequent statement would delete a PRIMARY KEY constraint named “PKArticle” using ALTER TABLE statement:

ALTER TABLE Article 
DROP CONSTRAINT PKArticle;

Also, check How To Use UNIQUE Constraint In SQL