PRIMARY KEY Keyword

Every row in a table is given a special identification by the PRIMARY KEY constraint.

A table can have only one primary key, which can be made up of one or more fields.

SQL PRIMARY KEY ON CREATE TABLE

Syntax

SQL
CREATE TABLE <TABLE_NAME>(
    <COLUMN_NAME> <DATATYPE> PRIMARY KEY,
    <COLUMN_NAME> <DATATYPE>,
    <COLUMN_NAME> <DATATYPE>
);

Example

SQL
CREATE TABLE Employee(
    Emp_id Integer PRIMARY KEY,
    Emp_Name Varchar(20),
    Emp_Gender Varchar(10),
    Dept_Id Integer
);

SQL PRIMARY KEY ON ALTER TABLE

Syntax

SQL
ALTER TABLE <TABLE_NAME> ADD PRIMARY KEY (<COLUMN_NAME>);

Example

SQL
ALTER TABLE Employee ADD PRIMARY KEY (Emp_id);

Summary

The PRIMARY KEY constraint uniquely identifies each row in a table.