unique constraint in sql code example

Example 1: how to drop a unique constraint in sql

SHOW CREATE TABLE [tableName]
-- Find the constraintID after CONSTRAINT 'constraintID'
ALTER TABLE [tableName] DROP FOREIGN KEY constraintID

Example 2: set column as unique in sql server

ALTER TABLE Persons
ADD UNIQUE (ID);

Example 3: unique constraint mssql

USE AdventureWorks2012;  
GO  
CREATE TABLE Production.TransactionHistoryArchive4  
 (  
   TransactionID int NOT NULL,   
   CONSTRAINT AK_TransactionID UNIQUE(TransactionID)   
);   
GO

Example 4: unique key in sql

Unique Key:
Only unique value and also can contain NULL