In below program only for first two rows it is working and for other rows it is showing incorrect values in plsql - oracle

Question

Here for the below program i need to print the amt_running_bal from the previous value. but it is not working and showing error. what is the error in the below program.Please provide any solution for this.
DECLARE
total Number := 1000000;
c_cod_acct_no Char;
c_amt_txn Number;
c_cod_drcr Char;
c_amt_running_bal Number;
amt_running_bal Number;
CURSOR c_chnos1 is
SELECT cod_drcr, amt_txn,amt_running_bal FROM chnos1;
BEGIN
OPEN c_chnos1;
FOR k IN 1..2 LOOP
FETCH c_chnos1 into c_cod_drcr,c_amt_txn,c_amt_running_bal;
if c_cod_drcr = 'C' then
total := total + c_amt_txn;
Update chnos1 SET amt_running_bal = total where cod_drcr='C' ;
elsif
c_cod_drcr = 'D' then
total := total - c_amt_txn;
Update chnos1 SET amt_running_bal = total where cod_drcr='D';
else
total := total + c_amt_txn;
Update chnos1 SET amt_running_bal = total where cod_drcr='C';
end if;
END LOOP;
CLOSE c_chnos1;
END;
/

Answer 1

Your query does not work as you limit the loop to k IN 1..2 so it will only read two rows from the cursor and there is no correlation between the row you are reading from the cursor and what you are updating; in fact, you are updating all the rows WHERE cod_drcr = 'C' or WHERE cod_drcr = 'D' and not just the current row. You could fix it by correlating the updates to the current row using the ROWID pseudo-column but it is an inefficient solution to use cursors as it will be slow and generate redo/undo log entries for each iteration of the cursor loop.
Instead, do it all in a single MERGE statement using an analytic SUM and a CASE expression:
MERGE INTO chnos1 dst
USING (
SELECT rowid AS rid,
1000000
+ SUM(
CASE cod_drcr
WHEN 'C' THEN +amt_txn
WHEN 'D' THEN -amt_txn
ELSE 0
END
)
OVER (
-- Use something like this to update each account
-- PARTITION BY cod_acct_no ORDER BY payment_date
-- However, you haven't said how to partition or order the rows so use this
ORDER BY ROWNUM
) AS total
FROM chnos1
) src
ON (dst.ROWID = src.rid)
WHEN MATCHED THEN
UPDATE SET amt_running_bal = src.total;
fiddle