define the field as unique sql server 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: unique constraint mssql

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