Oracle: How to count null and non-null rows

Question

I have a table with two columns that might be null (as well as some other columns). I would like to count how many rows that have column a, b, both and neither colu

Answer 1

select sum(decode(a,null,0,1)) as "NotNullCount", sum(decode(a,null,1,0)) as "NullCount"
from myTable;

Repeat for as many fields as you like.