Question : sql column that represents a unique tag number

We have a table with a column that represents a unique tag number [INumber].  We created a CREATE UNIQUE NONCLUSTERED INDEX on it and we want to add a contraint that will take the identify column as its default value.  How can we do this?

if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[ATable]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [dbo].[ATable]
GO

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE TABLE [dbo].[ATable](
      [ID] [int] IDENTITY(1,1) NOT NULL,
      [IName] [nvarchar](500) NOT NULL,
      [INumber] [nvarchar](50) NOT NULL,
 CONSTRAINT [PK_ATable] PRIMARY KEY CLUSTERED
(
      [ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 90) ON [PRIMARY]
) ON [PRIMARY]

GO


ALTER TABLE [dbo].[ATable] WITH NOCHECK ADD
      CONSTRAINT [DF_ATable_IName] DEFAULT ('') FOR [IName]
GO

 CREATE UNIQUE NONCLUSTERED INDEX [UIX_ATable_INumber] ON [dbo].[ATable]([INumber]) ON [PRIMARY]
GO

Answer : sql column that represents a unique tag number

You could use a TRIGGER for INSERT that updated this field to value of ID if it is NULL.