Taking Previous Months' Date Automatically in PLSQL - database

Question

Hello I have a procedure and questions about it. This procedure is used for extracting data then inserting them into one table. When I test my code, I have to enter some parameters for executing procedure.
`--this is how I execute the procedure
begin
GPU_DATA_EXTRACTOR(to_date('31/08/2021','DD/MM/YYYY'));
end;`
But what I want to do is that when the billdate parameter is NULL, the procedure should execute last day of the previous month as a parameter automatically. How can I make this change? I am open to any update advices thank you from now.
Updated the script below.
create or replace procedure GPU_DATA_EXTRACTOR_TEST(pid_billdate DATE DEFAULT LAST_DAY(ADD_MONTHS(TRUNC(SYSDATE), -1))) is
c_limit CONSTANT PLS_INTEGER DEFAULT 10000;
CURSOR c1 IS
SELECT DISTINCT intl_prod_id
FROM apld_bill_rt abr,
acct_bill ab
WHERE abr.CHRG_TP = 'INSTALLMENT'
AND abr.TAX_CATG_ID = 'NOTAX'
AND abr.acct_bill_id = ab.acct_bill_id
AND ab.bill_date = pid_billdate;
TYPE prod_ids_t IS TABLE OF apld_bill_rt.intl_prod_id%TYPE INDEX BY PLS_INTEGER;
l_prod_ids prod_ids_t;
begin
execute immediate 'truncate table GPU_INV_TEST';
OPEN c1;
LOOP
FETCH c1 BULK COLLECT INTO l_prod_ids LIMIT c_limit;
EXIT WHEN l_prod_ids.COUNT = 0;
FORALL indx IN 1 .. l_prod_ids.COUNT
INSERT INTO GPU_INV_TEST
SELECT AB.ACCT_BILL_ID,
AB.BILL_NO,
AB.INV_ID,
AB.BILL_DATE,
ba2.bill_acct_id,
ba1.bill_acct_id parent_bill_acct_id,
AB.DUE_DATE,
PG.CMPG_ID,
ABR.NET_AMT,
AB.DUE_AMT,
P.PROD_NUM,
pds.DST_ID,
ABR.DESCR,
p.intl_prod_id
FROM apld_bill_rt abr,
acct_bill ab,
prod p,
FCBSADM.PROD_DST pds,
bill_acct_prod bap,
bill_acct ba1,
bill_acct ba2,
prod_cmpg pg
WHERE ab.intl_bill_acct_id = ba1.intl_bill_acct_id
AND AB.ACCT_BILL_ID = ABR.ACCT_BILL_ID
AND ba1.intl_bill_acct_id = ba2.parent_bill_acct_id
AND ba2.intl_bill_acct_id = bap.intl_bill_acct_id
AND bap.intl_prod_id = abr.intl_prod_id
AND ABR.CHRG_TP = 'INSTALLMENT'
AND bap.intl_prod_id = pds.intl_prod_id
AND bap.intl_prod_id = p.intl_prod_id
AND p.intl_prod_id = pg.intl_prod_id(+)
AND ABR.intl_prod_id = l_prod_ids(indx)
UNION
SELECT AB.ACCT_BILL_ID,
AB.BILL_NO,
AB.INV_ID,
AB.BILL_DATE,
ba1.bill_acct_id,
ba1.bill_acct_id parent_bill_acct_id,
AB.DUE_DATE,
PG.CMPG_ID,
ABR.NET_AMT,
AB.DUE_AMT,
P.PROD_NUM,
pds.DST_ID,
ABR.DESCR,
p.intl_prod_id
FROM apld_bill_rt abr,
acct_bill ab,
prod p,
FCBSADM.PROD_DST pds,
bill_acct_prod bap,
bill_acct ba1,
prod_cmpg pg
WHERE ab.intl_bill_acct_id = ba1.intl_bill_acct_id
AND AB.ACCT_BILL_ID = ABR.ACCT_BILL_ID
--AND ba1.intl_bill_acct_id = ba2.parent_bill_acct_id
AND ba1.intl_bill_acct_id = bap.intl_bill_acct_id
AND bap.intl_prod_id = abr.intl_prod_id
AND ABR.CHRG_TP = 'INSTALLMENT'
AND bap.intl_prod_id = pds.intl_prod_id
AND bap.intl_prod_id = p.intl_prod_id
AND p.intl_prod_id = pg.intl_prod_id(+)
AND ABR.intl_prod_id = l_prod_ids(indx);
COMMIT;
END LOOP;
CLOSE c1;
end;

Answer 1

You can add a default value for your parameters. Take the following function as an example:
CREATE OR REPLACE FUNCTION sf_showDefault
(
p_in DATE DEFAULT LAST_DAY(ADD_MONTHS(TRUNC(SYSDATE), -1))
)
RETURN DATE
IS
BEGIN
RETURN p_in;
END sf_showDefault;
/
When no parameters are entered it gets a truncated SYSDATE and subtracts one month, then if finds the last day of that month. All the function does is return that data (or the one that you pass in...if you feel like it).
Here is a DBFiddle showing the effect of DEFAULT parameters (LINK)