SQL PRIMARY KEY Constraint

The PRIMARY KEY constraint is used to uniquely identify each row in a table. A PRIMARY KEY must contain unique values and it can’t contain a null value. A table can have only one primary key. We can use multiple columns or fields to define a primary key, such primary key is known as composite key.

Syntax of PRIMARY KEY Constraint on one column with CREATE TABLE statement:

MySQL:

CREATE TABLE Persons ( P_Id int NOT NULL, FirstName varchar(25) NOT NULL, LastName varchar(25), Address varchar(255), PRIMARY KEY (P_Id) )

SQL Server / Oracle / MS Access:

CREATE TABLE Persons ( P_Id int NOT NULL PRIMARY KEY, FirstName varchar(25) NOT NULL, LastName varchar(25), Address varchar(255), )

Syntax of PRIMARY KEY Constraint on one column with ALTER TABLE statement:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE CUSTOMER ADD PRIMARY KEY(ID);

Syntax of PRIMARY KEY Constraint on multiple columns with CREATE TABLE statement:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons ( P_Id int NOT NULL, FirstName varchar(25) NOT NULL, LastName varchar(25), Address varchar(255), PRIMARY KEY (P_Id, FirstName) )

Syntax of PRIMARY KEY Constraint on multiple columns with ALTER TABLE statement:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons ADD CONSTRAINT PK_PersonID PRIMARY KEY(P_Id, FirstName);

Delete PRIMARY KEY:

Use following syntax to delete the primary key.

MySQL:

ALTER TABLE Persons DROP PRIMARY KEY

SQL Server / Oracle / MS Access:

ALTER TABLE Persons DROP CONSTRAINT pk_PersonID