By Shivani Verma
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:-
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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 :-
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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 ;