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)
 );

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Output

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,
 );

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Output

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)
 );

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Output

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);

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Output

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);

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Output

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;

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Output

Example

 SQL Server / Oracle / MS Access:

 ALTER TABLE  student
 DROP CONSTRAINT PK_student;

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