Check SQL SYS_REFCURSOR without changing the cursor position - sql

Question

I have a requirement to check if a cursor was able to get some rows from table A. If yes, then do nothing else pull rows from table B.
Currently there are two stored procedures for now.
I am trying to do this in one stored procedure.
I tried using %ROWCOUNT but it doesn't work(because it will return 0) without changing the location of the cursor.
The issue is that my output is the cursor so I don't want to make any changes to.
If I do a fetch, then it shows error also that the return type has changed.
Any idea how to do this, like even if create a copy the cursor so that the fetch and row count could be done on the copy instead of the output cursor.
Pseudo Example
create or replace PROCEDURE "proc"
(
output OUT SYS_REFCURSOR,
)
.
BEGIN
.
.
OPEN output for select * from A
END
BEGIN
//check if output was empty then
OPEN output for select * from B
.
.
END
Update:
I did as suggested
....
BEGIN
...
OPEN output for
with
A as (select ...),
,B as (select ...),
,C as (select ...),
,D as (select ...)
select * from A
union
select * from B where not exists(select null from A)
union
select * from C where not exists(select null from B)
union
select * from D where not exists(select null from C)
END;
Since I know for sure that either one these tables will have data, I also tried the below
....
BEGIN
...
OPEN output for
with
A as (select ...)
,B as (select ...)
,C as (select ...),
,D as (select ...)
select * from A
union
select * from B
union
select * from C
union
select * from D
END;
But it gives me error now that
Error(64,7): PL/SQL: ORA-01789: query block has incorrect number of result columns
The table structure foe these 4 is diff. So they might return diff columns.
Would join make sense if 3 out of 4 are empty?

Answer 1

It's much better to implement your requirement in the same cursor, because it will use the same point-in-time read consistency: in your approach second open opens cursor for a different time than your first cursor and really data in A and B can change already.
This approach is better:
create or replace PROCEDURE "proc"( output OUT SYS_REFCURSOR,...)
....
BEGIN
...
OPEN output for
with
A as (select ...)
,B as (select ...)
select * from A
union all
select * from B where not exists(select null from A)
END;
Another possible solution is to create pipelined table instead, like:
create or replace function ... return {collection type} PIPELINED as
...flag boolean := true;
begin
....
for i in (select * from A) loop
flag:=false;
pipe row(...)
end loop;
if flag then
for i in (select * from B) loop
flag:=false;
pipe row(...)
end loop;
end if;
end;
/
But as you can see both query are opened at different time too.