single column filtering, same as IS NOT NULL or IS NULL in SQL for KDB Q -

t:flip `a`b`c`d`e!flip {5?(x;0N)} each til 10

select from t where e <> 0N
a b c d e
---------
3 3   3 3
    5 5 5
7 7   7 7
      8 8
9     9 9

select from t where e = 0N
a b c d e
---------
0 0 0 0
  1
    2
4   4
      6