How to replace a variable with another based on a condition in a stored procedure?

Question

I have the stored procedure below hat is working fine. I'd like, whoever, to be able to do a check to see if the dataset with YesterdayDate exists and, if not, replace YesterdayDate with TodayDate.

I was trying something like cheking if this count select count(1) from TableName where DATASET_DATE = YesterdayDate is 0, but I cannot find a way, assuming there is any, to add this check to the code without having to revamp or break everything else.

Stored Procedure:

CREATE OR REPLACE function DO_STUFF
(TodayDate date, YesterdayDate date)

return integer is status integer;

BEGIN 

DECLARE
  CURSOR c_stuff
  IS
    SELECT 
        <several_fields>
    FROM 
        TableName TODAY 
        LEFT JOIN TableName YESTERDAY
        ON  TODAY.KEY1 = YESTERDAY.KEY1
        WHERE TODAY.DATASET_DATE = TodayDate
        AND YESTERDAY.DATASET_DATE = YesterdayDate
    ORDER BY 
        TODAY.KEY1 ASC;
BEGIN
  FOR r_lines IN c_stuff
  LOOP
    <do_some_stuff>
  END LOOP;
END;

COMMIT;

STATUS := 0;

return status;

END
/

Does anyone have an idea on how this can be accomplished?

-- Edited to clarify: I do need to use both predicates. In the <do_some_stuff> section I'm mostly doing calculations and comparing the two datasets (Today versus yesterday) and updating some fields with the results. What I want to accomplish here is to deal with the situation where the Yesterday process was not ran and there is no yesterday dataset, by comparing today against today and avoiding incorrect data on the results.

Answer 1

I am not sure what you are trying to do with the function. Whatever I understood from your first line of question, You can try the below code -

CREATE OR REPLACE function DO_STUFF(TodayDate     date,
                                    YesterdayDate date)
RETURN INTEGER
IS
new_date DATE;
CNT NUMBER;
BEGIN
     SELECT COUNT(*)
       INTO CNT
       FROM TableName TODAY 
       LEFT JOIN TableName YESTERDAY ON TODAY.KEY1 = YESTERDAY.KEY1
      WHERE TODAY.DATASET_DATE = TodayDate
        AND YESTERDAY.DATASET_DATE = YesterdayDate
      ORDER BY TODAY.KEY1 ASC;
     IF CNT = 0 THEN
        RETURN TodayDate;
     ELSE
        RETURN YesterdayDate;
     END IF;
EXCEPTION
         WHEN OTHERS THEN
              RETURN '1900-01-01';
END;