Yakup KALEBAŞI

NULLIF kullanımı . . . 

CREATE TABLE #tmpTable (
  Id					int,
  CustomerName			varchar(50),
  identification_number varchar(11),
  tax_number			varchar(10)
  )
GO
 
INSERT #tmpTable VALUES(1,'Firma A', '12345678912', '9876543213');
INSERT #tmpTable VALUES(2,'Firma B', '', '4568529873');
INSERT #tmpTable VALUES(3,'Firma C', NUll, '9876543213');

select Id,
		CustomerName,
		isnull(identification_number, tax_number) as tax_number
from #tmpTable

select Id,
		CustomerName,
		isnull(nullif(identification_number,''), tax_number) as tax_number,
		'nullif KULLANILDIĞINDA' as info
from #tmpTable

--Farklı kullanımları var
--tax_number = '4568529873' ise NULL yapmak içinde kullanılabilir.
select Id,
		CustomerName,
		nullif(tax_number,'4568529873') as tax_number,
		'nullif KULLANILDIĞINDA' as info
from #tmpTable