SQL

primary key

CREATE TABLE Persons (
    Rollno int NOT NULL,
    FirstName varchar(255),
    LastName varchar(255),
    Age int,
    PRIMARY KEY (ID)
);

what is primary key

PRIMARY KEY  -- unique identifier for the entire row of record in a table 
             -- can not be null and must be unique 

primary key

Primary Key :
It is unique column in every table in a database
It can ONLY accept;
    - nonduplicate values
    - cannot be NULL