COALESCE( B~BSTKD, C~BSTKD ) AS BSTKD
.....
LEFT JOIN VBKD AS B
ON VBAP~VBELN = B~VBELN
AND VBAP~POSNR = B~POSNR
LEFT JOIN VBKD AS C
ON VBAP~VBELN = C~VBELN
AND C~POSNR = @GC_POSNR_HEAD
COALESCE( B~BSTKD, C~BSTKD ) AS BSTKD
.....
LEFT JOIN VBKD AS B
ON VBAP~VBELN = B~VBELN
AND VBAP~POSNR = B~POSNR
LEFT JOIN VBKD AS C
ON VBAP~VBELN = C~VBELN
AND C~POSNR = @GC_POSNR_HEAD