SQL Basic
SQL HOME SQL Intro SQL Syntax SQL Select SQL Distinct SQL Where SQL And & Or SQL Order By SQL Insert SQL Update SQL DeleteSQL Advanced
SQL Top SQL Like SQL Wildcards SQL In SQL Between SQL Alias SQL Joins SQL Inner Join SQL Left Join SQL Right Join SQL Full Join SQL Union SQL Select Into SQL Create DB SQL Create Table SQL Constraints SQL Not Null SQL Unique SQL Primary Key SQL Foreign Key SQL Check SQL Default SQL Create Index SQL Drop SQL Alter SQL Increment SQL Views SQL Dates SQL Nulls SQL isnull() SQL Data TypesSQL Functions
SQL Functions SQL avg() SQL count() SQL first() SQL last() SQL max() SQL min() SQL sum() SQL Group By SQL Having SQL ucase() SQL lcase() SQL mid() SQL len() SQL round() SQL now() SQL format()SQL Quick Ref SQL Hosting SQL Summary
SQL Quiz
SQL QuizSQL UNIQUE Constraint
SQL UNIQUE Constraint
The UNIQUE constraint uniquely identifies each record in a database table.
The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
SQL UNIQUE Constraint on CREATE TABLE
The following SQL creates a UNIQUE constraint on the "P_Id" column when the "Persons" table is created:
MySQL:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)
SQL Server / Oracle / MS Access:
CREATE TABLE Persons
(
P_Id int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
(
P_Id int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
)
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
)
SQL UNIQUE Constraint on ALTER TABLE
To create a UNIQUE constraint on the "P_Id" column when the table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ADD UNIQUE (P_Id)
ADD UNIQUE (P_Id)
To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
To DROP a UNIQUE Constraint
To drop a UNIQUE constraint, use the following SQL:
MySQL:
ALTER TABLE Persons
DROP INDEX uc_PersonID
DROP INDEX uc_PersonID
SQL Server / Oracle / MS Access:
ALTER TABLE Persons
DROP CONSTRAINT uc_PersonID
DROP CONSTRAINT uc_PersonID
W3Schools' Online Certification
The perfect solution for professionals who need to balance work, family, and career building.
More than 10 000 certificates already issued!
The HTML Certificate documents your knowledge of HTML.
The HTML5 Certificate documents your knowledge of advanced HTML5.
The CSS Certificate documents your knowledge of advanced CSS.
The JavaScript Certificate documents your knowledge of JavaScript and HTML DOM.
The jQuery Certificate documents your knowledge of jQuery.
The XML Certificate documents your knowledge of XML, XML DOM and XSLT.
The ASP Certificate documents your knowledge of ASP, SQL, and ADO.
The PHP Certificate documents your knowledge of PHP and SQL (MySQL).
Your suggestion:
Close [X]
Thank You For Helping Us!
Your message has been sent to W3Schools.
Close [X]
