The PRIMARY KEY keywords uniquely identifies each record in a table, which can have only one primary key consisting of a single field or multiple fields.

The following creates a PRIMARY KEY on the customer_id column when the Customers table is created.

MySQL:

CREATE TABLE Customers (
    customer_id int NOT NULL,
    last_name varchar(255) NOT NULL,
    first_name varchar(255),
    PRIMARY KEY (customer_id)
);

SQL Server:

CREATE TABLE Customers (
    customer_id int NOT NULL PRIMARY KEY,
    last_name varchar(255) NOT NULL,
    first_name varchar(255)
);

The following creates a PRIMARY KEY on the customer_id and last_name columns when the Customers table is created. It is one PRIMARY KEY made up of two columns.

MySQL and SQL Server:

CREATE TABLE Customers (
    customer_id int NOT NULL,
    last_name varchar(255) NOT NULL,
    first_name varchar(255),
    CONSTRAINT pk_customer PRIMARY KEY (customer_id, last_name)
);

The following creates a PRIMARY KEY on the customer_id column when the Customers table already exists.

MySQL and SQL: Server:

ALTER TABLE Customers
ADD PRIMARY KEY (customer_id);

The following creates a PRIMARY KEY on the customer_id and last_name columns when the Customers table already exists. It is one PRIMARY KEY made up of two columns.

MySQL and SQL Server:

ALTER TABLE Customers
ADD CONSTRAINT pk_customers PRIMARY KEY (customer_id, last_name);

MySQL:

ALTER TABLE Customers
DROP PRIMARY KEY;

SQL Server:

ALTER TABLE Customers
DROP CONSTRAINT pk_customer;

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