SQL PRIMARY KEY Constraint

ThePRIMARY KEY constraint uniquely identifies each record in a table.

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

A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

SQL PRIMARY KEY on CREATE TABLE

The following SQL creates a PRIMARY KEY on the "ID" column when the "student" table is created:

Example

MySQL

CREATE TABLE student(
id int NOT NULL ,
Last_name varchar( 50 ) NOT NULL ,
First_name varchar( 50 ) ,
Age int,
PRIMARY KEY (id)
);

Example

SQL Server / Oracle / MS Access:

CREATE TABLE student(
id int NOT NULL PRIMARY KEY ,
Last_name varchar( 50 ) NOT NULL ,
First_name varchar( 50 ) ,
Age int,
);

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

Example

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE student(
id int NOT NULL ,
Last_name varchar( 50 ) NOT NULL ,
First_name varchar( 50 ) ,
Age int,
CONSTRAINT PK_student PRIMARY KEY(id, Last_name)
);

Note: In the example above there is only ONE PRIMARY KEY (PK_student). However, the VALUE of the primary key is made up of TWO COLUMNS (id + Last_name).

SQL PRIMARY KEY on ALTER TABLE

The following SQL creates a PRIMARY KEY constraint on the "id" column when the table is already created, use the following SQL:

Example

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE student
ADD PRIMARY KEY (id);

To name a PRIMARY KEY constraint, and to define a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

Example

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE student
ADD CONSTRAINT PK_student PRIMARY KEY (id,Last_name);

NOTE: If you use ALTER TABLE to add a primary key, the primary key column(s) must have been declared to not contain NULL values (when the table was first created).

DROP a PRIMARY KEY Constraint

To drop a PRIMARY KEY constraint, use the following SQL:

Example

MySQL

ALTER TABLE student
DROP PRIMARY KEY;

Example

SQL Server / Oracle / MS Access:

ALTER TABLE student
DROP CONSTRAINT PK_student;