MySQL PRIMARY KEY Constraint

In MySQL, PRIMARY KEY constraint is used to ensure that all values in a column are unique and cannot contain NULL values. The PRIMARY KEY constraint forces the column to always accept a UNIQUE value without NULL. In MySQL, you can have only one PRIMARY KEY constraints on each table. The PRIMARY KEY constraint can be create on single column or group of multiple columns.

MySQL PRIMARY KEY Constraint example on create table

CREATE TABLE tblCustomer (
    custId int NOT NULL PRIMARY KEY,
    firstName varchar(255) NOT NULL,
    lastName varchar(255) NOT NULL,
    address varchar(255) NOT NULL
);

OR

CREATE TABLE tblCustomer (
    custId int NOT NULL,
    firstName varchar(255) NOT NULL,
    lastName varchar(255) NOT NULL,
    address varchar(255) NOT NULL,
	PRIMARY KEY (custId)
);

Note:

In the above example, we are creating new table called "tblCustomer" using PRIMARY KEY constraint. The PRIMARY KEY constraint enforces column "custId" to always accept UNIQUE value without NULL.

You can use MySQL Command Line Client to enforces column "custId" to always accept UNIQUE value without NULL.

MySQL PRIMARY KEY Constraint example on ALTER table

ALTER TABLE tblCustomer
ADD PRIMARY KEY (custId);

OR

ALTER TABLE tblCustomer
ADD CONSTRAINT PK_Customer PRIMARY KEY (custId,firstName);

MySQL DROP a PRIMARY KEY Constraint example

ALTER TABLE tblCustomer
DROP PRIMARY KEY;

OR

ALTER TABLE tblCustomer
DROP CONSTRAINT PK_Customer;