In MySQL, the PRIMARY KEY constraint is used to uniquely identify each record in a table.

Primary key contains UNIQUE values only and the column defined as Primary key column can not be NULL. A table can have only one PRIMARY KEY.

PRIMARY KEY Constraint on CREATE TABLE

The following statement creates a PRIMARY KEY constraint on the "id" column when the "employee" table is created:

Example

    CREATE TABLE employee
    (
    id int NOT NULL,
    first_name varchar(45) NOT NULL,
    last_name varchar(45),
    country varchar(45),
    PRIMARY KEY (id)
    );

We can also define PRIMARY KEY constraint on multiple column, for this use the below statement:

    CREATE TABLE emplyee
    (
    id int NOT NULL,
    first_name varchar(45) NOT NULL,
    last_name varchar(45),
    country varchar(45),
    CONSTRAINT pk_employeeId PRIMARY KEY (id,first_name)
    );

PRIMARY KEY Constraint on ALTER TABLE

We can also create PRIMARY KEY constraint on a column when the table is already created, for that we use ALTER command but the column must be declared as NOT NULL (When the table was created).

Syntax:

    ALTER TABLE table_name
    ADD PRIMARY KEY column_name

Example:

    ALTER TABLE employee
    ADD PRIMARY KEY (id);

Similarly we can define PRIMARY KEY constraint on multiple columns when the table is already created as below:

    ALTER TABLE employee
    ADD CONSTRAINT un_employeeId PRIMARY KEY (id,first_name);

To DROP a PRIMAR KEY Constraint

We can also drop a PRIMARY KEY constraint. To drop a PRIMARY KEY constraint, use the below statement:

    ALTER TABLE Persons
    DROP PRIMARY KEY;

Hope this will help you :)