IF NOT EXISTS (SELECT * FROM SYSCOLUMNS INNER JOIN SYSOBJECTS ON SYSOBJECTS.id = SYSCOLUMNS.id 
               WHERE SYSCOLUMNS.NAME = 'TUMCARILERDEGIRILIR' AND SYSOBJECTS.name = 'STOK')

ALTER TABLE dbo.STOK ADD TUMCARILERDEGIRILIR bit not null CONSTRAINT DF_STOK_TUMCARILERDEGIRILIR DEFAULT 1
go