CREATE TABLE [dbo].[testTable] ( [id] [int] IDENTITY (1, 1) NOT NULL , [nType] [tinyint] NOT NULL , [cnn] [char] (1) COLLATE Latin1_General_BIN NOT NULL , [checkunique] AS ([cnn] + case when ([nType] = 2) then (space(10)) else (convert(char(10),[id])) end) ) ON [PRIMARY] GO ALTER TABLE [dbo].[testTable] ADD CONSTRAINT [IX_testTable] UNIQUE NONCLUSTERED ( [checkunique] ) ON [PRIMARY]+ GOFabio