loop to sort internal table

hi,

i would like to know if this is possible.

i would like to sort internal table base on fieldX. if fieldX is initial then sort with condition as below else system default sort sequence.

i know the syntax incorrect. need advice with example.

loop at itab

if itab-fieldX is initial.

sort itab[] by fieldA ascending fieldB descending.

endif.

endloop.

thanks