title: How to design a mapping table?
body: <p>Im having a bit of a design issue here. Im kind of a novice in this, so I need some help.
  Due to a company merge, where everything must go in one of the systems; Im supposed to map our customers with a new customerid in the other company.</p>
  
  <p>When I get the new customerID's Im supposed to ensure that it is unique and the same goes for our existing customerID.</p>
  
  <pre><code>Current customerID: CurCustID
  New customer ID:    NewCustID
  </code></pre>
  
  <p>First, I would like the database to make sure that every CurCustID in column CurCustID is unique - only with one record, secondly I would like the column NewCustID to be unique - only with one record.
  Third I would like that the row combination of CurCustID and NewCustID only accepts unique data.</p>
  
  <p>If you can help me I would be very thankful, on the otherhand if my approach is bad practice and there is a best practice way of doing this, then please let me know.</p>
  
  <pre><code>USE [Database]
  GO
  
  /****** Object:  Table [dbo].[TblMapning]    Script Date: 05/30/2016 14:30:21 ******/
  SET ANSI_NULLS ON
  GO
  
  SET QUOTED_IDENTIFIER ON
  GO
  
  SET ANSI_PADDING ON
  GO
  
  CREATE TABLE [dbo].[TblMapning](
  [CurCustID] [varchar](255) NOT NULL,
  [NewCustID] [varchar](255) NOT NULL,
  PRIMARY KEY CLUSTERED 
  (
  [CurCustID] ASC
  )WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY =     OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
  ) ON [PRIMARY]
  
   GO
  
  SET ANSI_PADDING OFF
  GO
  </code></pre>