Basically Primary key is used to identify each row in a table . In a table Only one Primary key can be present . When in a table , there is more than one primary key then it would be called composite Primary key . Composite key is used when we need multiple column to identify a row .

Primary Key Constraints
1) Primary key always has a unique key i.e every Primary key is a unique key but vice-versa may not be true .
2) Primary Key can not accept Null value i.e Once you define Primary key in a table then you must enter the value .
3) In a table only one Primary key exist i.e no duplicate value for a Primary key .

Advantage :
Using Primary key we can access to the data faster .

Syntax in MySQL :

create table student ( s_id int not null , s_name varchar ( 20 ) not null Primary Key ( s_id ) ) ;

Syntax in SQL , Oracle , Ms Access :

create table student ( s_id int not null Primary Key , s_name varchar ( 20 ) not null ) ;

Syntax in MySQL , SQL , Oracle , Ms Access for Multiple Column :

create table student ( s_id int not null  , s_name varchar ( 20 ) not null , s_city varchar ( 30 ) constraint pk_studentID primary key ( s_id , s_name ) ) ;

Note : In above query there is only one Primary key ( pk_studentID ) and this primary key is made up of two column ( s_id and s_name ) ;

If you want to change existing Primary key in a table then you can use below code :

1) Primary key on one column :

alter table student add primary ( s_id ) ;

2) Primary key on multiple column :

alter table student add constraint pk_studentID primary key ( s_id , s_name ) ;

If you want to drop a Primary Key constraint then you can use below code :
In MySQL Database :

alter table student drop primary key ;

In SQL , Oracle , MS Access :

alter table student drop constraint pk_studentID ;