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.
![Primary key primary key](https://www.sitesbay.com/sql/images/primary-key.png)
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
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