SQL PRIMARY KEY Constraint (With Examples) (2024)

In SQL, the PRIMARY KEY constraint is used to uniquely identify rows. It is a combination of NOT NULL and UNIQUE constraints i.e. it cannot contain duplicate or NULL values.

Example

-- create Colleges table with primary key college_idCREATE TABLE Colleges ( college_id INT, college_code VARCHAR(20) NOT NULL, college_name VARCHAR(50), CONSTRAINT CollegePK PRIMARY KEY (college_id));

Here, the college_id column is the PRIMARY KEY. This means that the values of this column must be unique and it cannot contain NULL values.

Note that the above code works in all major database systems. However, some databases may have a different syntax.

SQL PRIMARY KEY Syntax

The syntax of the SQL PRIMARY KEY constraint is:

CREATE TABLE table_name ( column1 data_type, ..., [CONSTRAINT constraint_name] PRIMARY KEY (column1));

Here,

table_name is the name of the table to be created
column1 is the name of the column where the PRIMARY KEY constraint is to be defined
constraint_name is the arbitrary name given to the constraint
[...] signifies that the code inside is optional.

Note: Although naming a constraint using [CONSTRAINT constraint_name] is optional, doing so makes it easier to make changes to and delete the constraint.

Primary Key Error

In SQL, we will get an error If we try to insert NULL or duplicate values in the primary key column.

NOT NULL Constraint Error

We get this error when we supply the primary key with a NULL value. This is because primary keys need to obey the NOT NULL constraint. For example,

UNIQUE Constraint Error

We get this error when we supply duplicate values to the primary key, which violates its UNIQUE constraint. For example,

INSERT INTO Colleges(college_id, college_code, college_name)VALUES (1, "ARD12", "Star Public School");-- UNIQUE Constraint Error-- the value of college_id is not uniqueINSERT INTO Colleges(college_id, college_code, college_name)VALUES (1, "ARD12", "Star Public School");

Here, the SQL command gives us an error because we have inserted the duplicate value 1 into the primary key college_id.

Fix the UNIQUE Constraint Error

INSERT INTO Colleges(college_id, college_code, college_name)VALUES (1, "ARD12", "Star Public School");-- Insertion SuccessINSERT INTO Colleges(college_id, college_code, college_name)VALUES (2, "ARD12", "Star Public School");

Here, the SQL command runs without errors because we have supplied unique values 1 and 2 to college_id.

Note: There can only be one primary key in a table. However, that single primary key can contain multiple columns.

Primary Key With Multiple Columns

Primary Key Constraint With Alter Table

We can also add the PRIMARY KEY constraint to a column in an existing table using the ALTER TABLE command. For example,

For a Single Column

ALTER TABLE CollegesADD PRIMARY KEY (college_id);

Here, the SQL command adds the PRIMARY KEY constraint to the college_id column in the existing Colleges table.

For Multiple Columns

ALTER TABLE CollegesADD CONSTRAINT CollegePK PRIMARY KEY (college_id, college_code);

Here, the SQL command adds the PRIMARY KEY constraint to the college_id and college_id columns in the existing Colleges table.

Note: This command is not supported by our online SQL editor as it is based on SQLite.

Auto Increment Primary Key

It is a common practice to automatically increase the value of the primary key whenever a new row is inserted. For example,

SQL Server

-- use IDENTITY(x, y) to auto increment the value-- x -> start value, y -> steps to increaseCREATE TABLE Colleges ( college_id INT IDENTITY(1,1), college_code VARCHAR(20) NOT NULL, college_name VARCHAR(50), CONSTRAINT CollegePK PRIMARY KEY (college_id));-- insert record without college_idINSERT INTO Colleges(college_code, college_name)VALUES ("ARD13", "Star Public School");

Oracle

-- create sequence of numbersCREATE SEQUENCE auto_incMINVALUE 1START WITH 1INCREMENT BY 1CACHE 10;CREATE TABLE Colleges ( college_id INT, college_code VARCHAR(20) NOT NULL, college_name VARCHAR(50), CONSTRAINT CollegePK PRIMARY KEY (college_id));-- create trigger before insert to-- add auto incremented valueCREATE TRIGGER auto_inc_triggerBEFORE INSERT ON CollegesFOR EACH ROWBEGINSELECT auto_inc.nextval INTO :new.college_id FROM dualEND;-- insert record without college_idINSERT INTO Colleges(college_code, college_name)VALUES ("ARD13", "Star Public School");

MySQL

-- AUTO_INCREMENT keyword auto increments the valueCREATE TABLE Colleges ( college_id INT AUTO_INCREMENT, college_code VARCHAR(20) NOT NULL, college_name VARCHAR(50), CONSTRAINT CollegePK PRIMARY KEY (college_id));-- insert record without college_idINSERT INTO Colleges(college_code, college_name)VALUES ("ARD13", "Star Public School");

PostgreSQL

-- SERIAL keyword auto increments the valueCREATE TABLE Colleges ( college_id INT SERIAL, college_code VARCHAR(20) NOT NULL, college_name VARCHAR(50), CONSTRAINT CollegePK PRIMARY KEY (college_id));-- insert record without college_idINSERT INTO Colleges(college_code, college_name)VALUES ("ARD13", "Star Public School");

Remove Primary Key Constraint

We can remove the PRIMARY KEY constraint in a table using the DROP clause. For example,

SQL Server, Oracle

ALTER TABLE CollegesDROP CONSTRAINT CollegePK;

MySQL

ALTER TABLE CollegesDROP PRIMARY KEY;

Here, the SQL command removes the PRIMARY KEY constraint from the Colleges table.

Also Read:

SQL CREATE TABLE
SQL INSERT INTO
SQL ALTER TABLE
SQL FORIEGN KEY
SQL Constraints