SQL PRIMARY KEY Constraint

1. The PRIMARY KEY constraint uniquely identifies each record in a database table.


2. Primary keys must contain UNIQUE values.

3.A primary key column cannot contain NULL values.

4. Most tables should have a primary key, and each table can have only ONE primary key.

The following SQL creates a PRIMARY KEY on the "P_Id" column when the "Persons" table is created:

CREATE TABLE Persons
(
P_Id int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)

To create a PRIMARY KEY constraint on the "P_Id" column when the table is already created, use the following SQL:

ALTER TABLE Persons
ADD PRIMARY KEY (P_Id)

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

ALTER TABLE Persons
ADD CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)

To drop a PRIMARY KEY constraint, use the following SQL:

ALTER TABLE Persons
DROP CONSTRAINT pk_PersonID