UPDATE c

SET c.SoDiagnosisID =  a.SoDiagnosisID

from inv_schmerge c

inner join DiagnosticProc_DS b on

 b.DiagnosticProcProcID = c.Diagnosticprocid

inner join Diagnosis_ds a on

 a.DiagnosisName=b.DiagnosisName

where c.sodiagnosisid is null

From this example you can see that I am updating...