table1 ,aa 14, 12,bb
table2 ,aa 14, 12,bb k = cogroup table1 by (a,b),table2 by (a,b); k: ((,aa),{(,aa)},{}) ((,aa),{},{(,aa)}) ((14,),{(14,)},{(14,)}) ((12,bb),{(12,bb)},{(12,bb)}) when I use cogroup,(,aa) null is in front,(14,) null is not in front. is there a parameter I can get result like this ((,aa),{(,aa)},{(,aa)}) ((14,),{(14,)},{(14,)}) ((12,bb),{(12,bb)},{(12,bb)}) thank you.