Primary key uniquely identifies each row in a table. It consist of a single column or combination of multiple columns in a table. Primary key column must be unique and can not be NULL.
PRIMARY KEY Constraint can be defined while creating the table or afterward also.
Example:
Single column Primary Key
CREATE TABLE EMPLOYEE (
EMP_ID int NOT NULL,
EMP_NAME varchar(30),
GRADE varchar(20),
PRIMARY KEY (EMP_ID)
);
Multi-column Primary Key
CREATE TABLE EMPLOYEE (
EMP_ID int NOT NULL,
EMP_SSN int NOT NULL,
EMP_NAME varchar(30),
GRADE varchar(20),
PRIMARY KEY (EMP_ID, EMP_SSN)
);
Creation of Primary Key After Table Creation
ALTER TABLE EMPLOYEE
ADD CONSTRAINT PKey_Emp PRIMARY KEY (EMP_ID, EMP_SSN);
Dropping of Primary Key
ALTER TABLE EMPLOYEE
DROP CONSTRAINT Pkey_Emp