Tutorial on SQL UNIQUE constraints

SQL statements with unique constraints
The unique constraint uniquely identifies each record in a database table.

Both the uniqueness and primary constraints provide a unique column or a group of columns.

A primary key constraint automatically defines it with a unique constraint.

Note that you can have many unique restrictions on each table, but only one primary key constraint per seat.


Unique SQL constraints for creating tables
When the following SQL statement creates a column named "P_Id" with a unique constraint, "person" creates a table:

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))

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))
 
To name a unique constraint and determine the unique constraint on multiple columns, use the following SQL Syntax:
For 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))
Unique constraints of Database Change table
To create a column named "P_Id" with a unique constraint, the table has been created. Use the following SQL statement:
ALTER TABLE PersonsADD UNIQUE (P_Id)
 
To name a unique constraint and determine the unique constraint on multiple columns, use the following SQL Syntax:
 
ALTER TABLE PersonsADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
 
Drop unique is a unique constraint
To discard a unique constraint, use the following SQL:
 
ALTER TABLE PersonsDROP INDEX uc_PersonID
 
SQL Server / Oracle / MS Access:
ALTER TABLE PersonsDROP CONSTRAINT uc_PersonID