oracle cursor value vary depending on condition - sql

Question

I would like to ask if it is possible to assign a variable on a cursor declaration.
CURSOR cur_name IS <variable_name>
What I would like to accomplish is that in a cursor some of where clause and the from clause of the select statement varies depending on the result of another select. Like below:
select count(*) from table_name
v_cnt
where cond1;
IF v_cnt is 0, cursor would be:
cursor cur_name IS
select * from tab_name1
where cond1;
IF v_cnt > 0, cursor would be:
cursor cur_name IS
select * from tab_name2
where cond1
and cond2;
I was wondering if I can do an if-else and then concat on the select that would be assign on the cursor.
cursor cur_name IS
select * from tab_name
if v_cnt > 0
where cond2;
else
where cond1;
Let me know if you need more details.
Appreciate any feedback.

Answer 1

Why not use something like
select *
from tab_name
WHERE (v_cnt = 0 AND cond1)
OR (v_cnt > 0 AND cond2)

Answer 2

Are you looking for something like this ?
DECLARE
V_CNT VARCHAR2(20);
CURSOR C1
IS
SELECT * from Tab1;
CURSOR C2
IS
SELECT * from Tab2;
BEGIN
SELECT COUNT(*) INTO V_CNT FROM Table_Name;
IF V_CNT > 0 THEN
OPEN C1;
--code
Close C1;
ELSE
OPEN C2;
--code
CLOSE C2;
END IF;
END;

Answer 3

If the cursor is very dynamic use something like:
declare
c sys_refcursor;
<here declare the record you would like fetch results to>
begin
open c for 'you query in quotes as the string that you created before regarding your conditions';
loop
FETCH c INTO your record;
EXIT WHEN c%NOTFOUND;
end loop;
end;
At any case have a look into http://docs.oracle.com/cd/B19306_01/appdev.102/b14261/sqloperations.htm#BABFEJED. It describes your case by my opinion.

Answer 4

There are a lot of ways to do this, and there are many trade-offs involved.
Alexander Tokarev's answer is the most flexible. But dynamic SQL can be tricky, dependency problems won't show up at compile time, etc. Balaji Sukumaran's answer is less flexible, but simpler, and breaks the code into smaller chunks.
If the columns selected will always be the same, you can use a method like this:
cursor cur_name(v_cnt number) is
select *
from tab_name1
where 1=1 /*condition 1*/
and v_cnt > 0
---------
union all
---------
select *
from tab_name2
where 2=2 /*condition 2*/
and (v_cnt is null or v_cnt <= 0);
It puts everything together, which can be more confusing than Balaji's answer. But sometimes it's better to have all the logic in a single SQL. It may help reduce repeating logic.
(Also, you don't need to worry about Oracle actually using both queries, and running slowly. It's smart enough to know that there's a bind variable that controls which query is used. That's what the FILTER step in the explain plan does.)