How did you verify that? Is it just because you got a rows affected of 1? Perhaps there was only one row that met the condition?
What do you get when you run your original query of...
SELECT E.[Column 0] FROM dbo.abn_extract E JOIN dbo.ABN_File_Header H ON
SUBSTRING(E.[Column 0], 1, 13) = H.Header_Id
Sometimes, the devil is in the data... please read and heed the link in my signature line to get better help quicker. Thanks, Neal.
--Jeff Moden
Change is inevitable... Change for the better is not.