how to add unique constraint in sql code example

Example 1: sql server add unique constraint

-- Add a Unique constraint and deifine the constraint name
ALTER TABLE TableName
ADD	CONSTRAINT TableName_ColumnName_Unique UNIQUE (ColumnName)

Example 2: how to add unique constraint in mysql table

ALTER TABLE mytbl ADD UNIQUE (columnName);

Example 3: 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 4: set column as unique in sql server

ALTER TABLE Persons
ADD UNIQUE (ID);

Example 5: unique constraint mssql

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