SQL Primary Key

SQL Primary Key Constraints

Primary Key Constraints does not allow to enter duplicate as well as null values. This constraints is used to uniquely identify the record. You can not apply Primary Key constraints on more than one column in same table.

Advantage of Primary Key Constraints

Primary Key constraints does not allows to enter duplicate as well as null value.

Dis-Advantage of Primary Constraints

You can not apply this constraints on more than one column in same table.

SQL Server / Oracle / MS Access:

 
create table employee
(
e_id number(3) primary key,
ename varchar2(15),
sal number(5)
);

MySQL

 
create table employee
(
e_id number(3) primary key,
ename varchar(15),
sal number(5),
primary key (e_id)
);

primary key constraint on ALTER TABLE:

This is the way to add unique key constraint on any column of table which table is already created, use following syntax;

MySQL / SQL Server / Oracle / MS Access:

 
ALTER TABLE employee
ADD primary key (E_Id)

Drop Primary Key

MySQL

 
alter table employee
drop primary key

SQL Server / Oracle / MS Access:

 
ALTER TABLE employee
DROP CONSTRAINT pk_employeeID