CodesJava

Easy learning with example program codes

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

 
Next Topic: SQL FOREIGN KEY with example.
Previous Topic: SQL CONSTRAINTS with example.

Related Topics:

How to build java project using ant in eclipse?
JAXB marshalling – convert java object to xml example.
How to create pdf file in java using iText jar?
Generics class example.
OGNL in struts 2.
Hibernate One-to-One Mapping using xml.
Send inline image in email using JavaMail API.
Quartz 2 JobListener example.

 

Posted in SQL   
Sign Up/ Sign In
Ask a Question


Industrial Training

We offers Placement Oriented Training on Java, Spring, JSF, Hibernate, PHP, AngularJS, Angular 4, PLSQL, Oracle BI Publisher etc. We also provides Online training, please mail us at hr@codesjava.com.

Development

We also provides the Development services for Website Development , Java Development, PHP Development, Android App Development etc. You can contact us on hr@codesjava.com.

Copyright © 2018 CodesJava DMCA.com Protection Status SiteMap Reference: Java Wiki