Select 
	CASE 
		When Qualifer='PL' and Segment='SLN' Then 'Line Number'
		When Qualifer='DV' and Segment='SLN' Then 'Department'
		When Qualifer='91' and Segment='PID' Then 'Size'
		ELSE NULL
	END VALUE 
From
	aTable

SELECT col1
, col2
, CASE WHEN (col1 + col2) = 42 THEN 'Department' ELSE 'somethingelse' END
FROM myTable

CASE ValuetoTest
  WHEN SomeValue
  THEN SomeOtherValue
  ELSE DefaultValue
END
--OR 
CASE
WHEN {SomeExpression} (ie SomeColumn = SomeValue)
THEN SomeOtherValue
ELSE DefaultValue
END

SELECT
  zzoordrh.BILL_NUM,
  zzedatawhse.value,
  CASE
    WHEN zzedatawhse.QUALIFIER = 'PL'
         AND zzedatawhse.SEGMENT = 'SLN'
    THEN 'Line No'
    WHEN zzedatawhse.QUALIFIER = 'DV'
         AND zzedatawhse.SEGMENT = 'SLN'
    THEN 'Department'
    WHEN zzedatawhse.QUALIFIER = '91'
         AND zzedatawhse.SEGMENT = 'PID'
    THEN 'Line No'
  END AS [zzedatawhse.value]
FROM   zzedatawhse
       INNER JOIN zzoordrh
         ON zzedatawhse.DOC_NUM = zzoordrh.edi_doc_num
WHERE  zzoordrh.BILL_NUM = '113111'