What is primary key

How to make multiple keys as primary in bdms

CREATE TABLE `Donor`(
  `did` int(50) NOT NULL,
  `oid` int(50) NOT NULL,
  `dname` varchar(50) NOT NULL,
  `dblood` varchar(50) NOT NULL,
  PRIMARY KEY (`did`, `oid`)
);

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

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

Source: computerscienceai.com

Primary key

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

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