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 columns set to null.

Is this possible with Oracle in one query? Or would I have to create one query for each? Can't use group by or some other stuff I might not know about for example?

Answer 1

COUNT(expr) will count the number of rows where expr is not null, thus you can count the number of nulls with expressions like these:

SELECT count(a) nb_a_not_null,        count(b) nb_b_not_null,        count(*) - count(a) nb_a_null,        count(*) - count(b) nb_b_null,        count(case when a is not null and b is not null then 1 end)nb_a_b_not_null        count(case when a is null and b is null then 1 end) nb_a_and_b_null   FROM my_table 

Answer 2

Something like this:

 SELECT sum(case                 when a is null and b is null then 1                else 0            end) as both_null_count,        sum(case                when a is null and b is not null then 1                else 0            end) as only_a_is_null_count FROM your_table 

You can extend that for other combinations of null/not null