Counting two different values for each year - sql

Question

I have a table which includes Years and Genders. Here is an example of my table:
YEAR Sex
1999 M
1999 M
1999 F
1999 F
I use this query for taking my result.
SELECT YEAR,COUNT(*)
FROM athlete_events
WHERE SEX = 'M'
GROUP BY YEAR;
And I see this result on output:
YEAR COUNT(*)
1999 2
However I want to see this result:
YEAR COUNT_MALE COUNT_FEMALE
1999 2 2
Is that possible in Pracle SQL?

Answer 1

You can do conditional aggregation with a case expression:
select year,
count(case when sex = 'M' then year end) as count_male,
count(case when sex = 'F' then year end) as count_female
from athlete_events
group by year;
YEAR COUNT_MALE COUNT_FEMALE
---------- ---------- ------------
1999 2 2
The count function ignores nulls, so rows that don't match the specified flag (which default to null in the case expression; you can have an explicit else null if you prefer) are not counted.
(Personally I prefer to use count() rather than sum() for this sort of thing as that better reflects what you're actually doing - counting things.)

Answer 2

You need conditional aggregation :
select year,
sum(case when SEX = 'M' then 1 else 0 end) as count_male,
sum(case when SEX = 'F' then 1 else 0 end) as count_female
from athlete_events
group by year;

Answer 3

You can do subqueries, something like this:
SELECT
E.YEAR,
(select count(*) from athlete_events M where M.YEAR=E.YEAR and M.SEX='M') COUNT_MALE,
(select count(*) from athlete_events F where F.YEAR=E.YEAR and F.SEX='F') COUNT_FEMALE
FROM athlete_events E
GROUP BY E.YEAR;
This is more expensive (performance-wise) than conditional aggregation, on the other hand maybe easier to understand and maintain.