How do I conditionally group by two different columns in Oracle? - oracle

Question

Suppose I have a table with the following data:
+----------+-----+--------+
| CLASS_ID | Day | Period |
+----------+-----+--------+
| 1 | A | CCR |
+----------+-----+--------+
| 1 | B | CCR |
+----------+-----+--------+
| 2 | A | 1 |
+----------+-----+--------+
| 2 | A | 2 |
+----------+-----+--------+
| 3 | A | 3 |
+----------+-----+--------+
| 3 | B | 4 |
+----------+-----+--------+
| 4 | A | 5 |
+----------+-----+--------+
As you could probably guess from the nature of the data, I'm working on an Oracle SQL query that pulls class schedule data from a Student Information System. I'm trying to pull a class's "period expression", a calculated value that contains the Day and Period fields into a single field. Let's get my expectation out of the way first:
If the Periods match, Period should be the GROUP BY field, and Day should be the aggregated field (via a LISTAGG function), so the calculated field would be CCR (A-B)
If the Days match, Day should be the GROUP BY field, and Period should be the aggregated field, so the calculated field would be 1-2 (A)
I'm only aware of how to do each GROUP BY individually, something like for where Days match:
SELECT
day,
LISTAGG(period, '-') WITHIN GROUP (ORDER BY period)
FROM schedule
GROUP BY day
and vice versa for matching Periods, but I'm not seeing how I could do that dynamically for Period and Day in the same query.
You'll also notice that the last row in the example data set doesn't span multiple days or periods, so I also need to account for classes that don't need a GROUP BY at all.
Edit
The end result should be:
+------------+
| Expression |
+------------+
| CCR(A-B) |
+------------+
| 1-2(A) |
+------------+
| 3-4(A-B) |
+------------+
| 5(A) |
+------------+

Answer 1

It is really not clear to me WHY you want output in that way. It doesn't provide any useful information (I don't think) - you can't tell, for example for class_id = 3, which combinations of day and period are actually used. There are four possible combinations (according to the output), but only two are actually in the class schedule.
Anyway - you may have your reasons. Here is how you can do it. You seem to want to LISTAGG both the day and the period (both grouped by class_id, they are not grouped by each other). The difficulty is that you want distinct values in the aggregate lists only - no duplicates. So you will need to select distinct, separately for period and for day, then to the list aggregations, and then concatenate the results in an inner join.
Something like this:
with
test_data ( class_id, day, period ) as (
select 1, 'A', 'CCR' from dual union all
select 1, 'B', 'CCR' from dual union all
select 2, 'A', '1' from dual union all
select 2, 'A', '2' from dual union all
select 3, 'A', '3' from dual union all
select 3, 'B', '4' from dual union all
select 4, 'A', '5' from dual
)
-- end of test data; the actual solution (SQL query) begins below this line
select a.class_id, a.list_per || '(' || b.list_day || ')' as expression
from ( select class_id,
listagg(period, '-') within group (order by period) as list_per
from ( select distinct class_id, period from test_data )
group by class_id
) a
inner join
( select class_id,
listagg(day, '-') within group (order by day) as list_day
from ( select distinct class_id, day from test_data )
group by class_id
) b
on a.class_id = b.class_id
;
CLASS_ID EXPRESSION
-------- ----------
1 CCR(A-B)
2 1-2(A)
3 3-4(A-B)
4 5(A)

Answer 2

How about union with having count(*) = 1?
select LISTAGG(period, '-') list WITHIN GROUP (ORDER BY period)
from schedule
group by CLASS_ID, day
having count(*) = 1
union all
select LISTAGG(day, '-') list WITHIN GROUP (ORDER BY day)
from schedule
group by CLASS_ID, period
having count(*) = 1