Putting values into a collection for different date ranges - oracle

Question

I am writing a PL/SQL procedure which gives the count of a query based on date range values. I want to get the date range dynamically and I have written a cursor for that.
I am using a collection and getting the counts of each month, the problem I am facing is that collection is populated with the count of the last month alone. I want to get the count of all months. Can anyone help?
This is the procedure I have written:
create or replace
Procedure Sample As
Cursor C1 Is
With T As (
select to_date('01-JAN-17') start_date,
Last_Day(Add_Months(Sysdate,-1)) end_date from dual
)
Select To_Char(Add_Months(Trunc(Start_Date,'mm'),Level - 1),'DD-MON-YY') St_Date,
to_char(add_months(trunc(start_date,'mm'),level),'DD-MON-YY') ed_date
From T
Connect By Trunc(End_Date,'mm') >= Add_Months(Trunc(Start_Date,'mm'),Level - 1);
Type T_count_Group_Id Is Table Of number;
V_count_Group_Id T_count_Group_Id;
Begin
For I In C1
Loop
Select Count(Distinct c1) bulk collect Into V_Count_Group_Id From T1
Where C2 Between I.St_Date And I.Ed_Date;
End Loop;
For J In V_Count_Group_Id.First..V_Count_Group_Id.Last
Loop
Dbms_Output.Put_Line(V_Count_Group_Id(J));
end loop;
END SAMPLE;

Answer 1

Your bulk collect query is replacing the contents of the collection each time around the loop; it doesn't append to the collection (if that's what you expected). So after your loop you are only seeing the result of the last bulk collect, which is the latest month from your cursor.
You're also apparently comparing dates as string, which isn't a good idea (unless c2 is stored as a string - which is even worse). And as between is inclusive, you risk including data for the first day of each month in two counts, if the stored time portion is midnight. It's safer to use equality checks for date ranges.
You don't need to use a cursor to get the dates and then individual queries inside that cursor, you can just join your current cursor query to the target table - using an outer join to allow for months with no matching data. Your cursor seems to be looking for all months in the current year, up to the start of the current year, so that could perhaps be simplified to:
with t as (
select add_months(trunc(sysdate, 'YYYY'), level - 1) as st_date,
add_months(trunc(sysdate, 'YYYY'), level) as ed_date
from dual
connect by level < extract(month from sysdate)
)
select t.st_date, t.ed_date, count(distinct t1.c1)
from t
left join t1 on t1.c2 >= t.st_date and t1.c2 < t.ed_date
group by t.st_date, t.ed_date
order by t.st_date;
You can use that to populate your collection:
declare
type t_count_group_id is table of number;
v_count_group_id t_count_group_id;
begin
with t as (
select add_months(trunc(sysdate, 'YYYY'), level - 1) as st_date,
add_months(trunc(sysdate, 'YYYY'), level) as ed_date
from dual
connect by level < extract(month from sysdate)
)
select count(distinct t1.c1)
bulk collect into v_count_group_id
from t
left join t1 on t1.c2 >= t.st_date and t1.c2 < t.ed_date
group by t.st_date, t.ed_date
order by t.st_date;
for j in v_count_group_id.first..v_count_group_id.last
loop
dbms_output.put_line(v_count_group_id(j));
end loop;
end;
/
although as it only stores/shows the counts, without saying which month they belong to, that might not ultimately be what you really need. As the counts are ordered, you at least know that the first element in the collection represents January, i suppose.