PRIMARY KEY IN SQL

PRIMARY-KEY - SQL

A primary key is a field in a table which uniquely identifies each row/record in a database table. Primary keys must contain unique values. A primary key column cannot have NULL values.

A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key.

If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s).

To define PRIMARY KEY in SQL:-

  CASE 1:-When making the table
  syntax-
    CREATE TABLE STUDENTS(
       ROLL_NO INT             NOT NULL,
       NAME VARCHAR (20)     NOT NULL,
       AGE  INT              NOT NULL,   
       PRIMARY KEY (ROLL_NO)
    );

  CASE 2:-When there is an existing table in which there is no primary key and you  want to add primary key
  syntax -
    (write the following command)
    ALTER TABLE STUDENTS ADD PRIMARY KEY (ROLL_NO);

  CASE 3 :- For defining a PRIMARY KEY constraint on multiple columns
  syntax -
    CREATE TABLE STUDENTS(
     ROLL_NO   INT              NOT NULL,
     NAME VARCHAR (20)     NOT NULL,
     AGE  INT              NOT NULL,
     PHONE_NO INT (10)      
     PRIMARY KEY (ROLL_NO,PHONE_NO)
  );

To delete a PRIMARY KEY :-

  APPLICABLE FOR ALL CASES:-
  syntax -
     (You can clear the primary key constraints from the table with the syntax given below)
     ALTER TABLE STUDENTS DROP PRIMARY KEY ;

databases

Primary_key, implementation, Composite_key