Şunu bi deneyebilirmiyiz?
SELECT SOURCECOSTGRP, IOCODE, SUM(AMOUNT)
FROM LG_001_01_STLINE STLINE
LEFT JOIN LG_001_ITEMS ITEMS ON ITEMS.LOGICALREF=STLINE.STOCKREF
WHERE CANCELLED=0
AND ITEMS.CODE='HL 10396'
GROUP BY SOURCECOSTGRP, IOCODE
ORDER BY SOURCECOSTGRP, IOCODE