Packaged Procedure calling private function - unusable in SQL - oracle

Question

I have a package with two private functions, and one procedure which calls them. The two functions return due dates (first, and last) of pledged payments to the procedure. The procedure returns those dates, along with the name, and ID of the donor.
CREATE OR REPLACE PACKAGE PLEDGE_PKG IS
PROCEDURE DD_PLIST_PP
(p_id IN DD_PLEDGE.IDPLEDGE%TYPE,
p_first DD_DONOR.FIRSTNAME%TYPE,
p_last DD_DONOR.LASTNAME%TYPE,
p_payfirst OUT DATE,
p_paylast OUT DATE);
END;
CREATE OR REPLACE PACKAGE BODY PLEDGE_PKG IS
-- Determines 1st Payment Due Date based on ID.
FUNCTION dd_paydate1_pf
(p_id IN dd_pledge.idpledge%TYPE)
RETURN DATE
IS
lv_pl_dat DATE;
lv_mth_txt VARCHAR2(2);
lv_yr_txt VARCHAR2(4);
BEGIN
SELECT ADD_MONTHS(pledgedate,1)
INTO lv_pl_dat
FROM dd_pledge
WHERE idpledge = p_id;
lv_mth_txt := TO_CHAR(lv_pl_dat,'mm');
lv_yr_txt := TO_CHAR(lv_pl_dat,'yyyy');
RETURN TO_DATE((lv_mth_txt || '-01-' || lv_yr_txt),'mm-dd-yyyy');
END dd_paydate1_pf;
-- Determines LAST Payment Due Date based on ID.
FUNCTION dd_payend_pf
(p_id IN dd_pledge.idpledge%TYPE)
RETURN DATE
IS
lv_pay1_dat DATE;
lv_mths_num dd_pledge.paymonths%TYPE;
BEGIN
SELECT dd_paydate1_pf(idpledge), paymonths - 1 -- LINE 28
INTO lv_pay1_dat, lv_mths_num
FROM dd_pledge
WHERE idpledge = p_id;
IF lv_mths_num = 0 THEN
RETURN lv_pay1_dat;
ELSE
RETURN ADD_MONTHS(lv_pay1_dat, lv_mths_num);
END IF;
END dd_payend_pf;
-- Displays Donor Name, ID, First, Last payment using Donor ID
PROCEDURE DD_PLIST_PP
(p_id IN DD_PLEDGE.IDPLEDGE%TYPE,
p_first DD_DONOR.FIRSTNAME%TYPE,
p_last DD_DONOR.LASTNAME%TYPE,
p_payfirst OUT DATE,
p_paylast OUT DATE)
AS
lv_first DD_DONOR.FIRSTNAME%TYPE;
lv_last DD_DONOR.LASTNAME%TYPE;
BEGIN
SELECT FIRSTNAME, LASTNAME
INTO lv_first, lv_last
FROM DD_DONOR
WHERE p_id = IDDONOR;
p_payfirst := PLEDGE_PKG.DD_PAYDATE1_PF(p_id);
p_paylast := PLEDGE_PKG.DD_PAYEND_PF(p_id);
END DD_PLIST_PP;
END;
The error I am getting is from the second block.
LINE/COL ERROR
-------- -----------------------------------------------------------------
28/3 PL/SQL: SQL Statement ignored
28/10 PL/SQL: ORA-00904: : invalid identifier
28/10 PLS-00231: function 'DD_PAYDATE1_PF' may not be used in SQL
SO requests more text and less code. :)
My issue is I want to call the PLEDGE_PKG.DD_PLIST_PP and get back the name, and the two dates back.
Hopefully this explanation helps, and thank you for the help.

Answer 1

As you marked it, line #28 is
SELECT dd_paydate1_pf(idpledge), paymonths - 1
This statement is SQL, and you use it within the package (which is PL/SQL). SQL engine can't access function which is private to that package, unless you make it public. Therefore, what would work is
what you used in line #55:
p_payfirst := PLEDGE_PKG.DD_PAYDATE1_PF(p_id);
as here everything is in PL/SQL (though, that's questionable as you're passing P_ID parameter's value from another table in line #28), or
make the function public by declaring it in package specification
[EDIT]
Based on your comment (too much text for replying through another comment):
It is about context switching. select dd_paydate1_pf ... is SQL called from PL/SQL. SQL tries to select function's result and can't find that function at the SQL layer. If you preceded it with package name, i.e. select pledge_pkg.dd_paydate1_pf..., it still wouldn't work as that function isn't declared in package specification but is private to package body.
What you could do is to rewrite function as:
FUNCTION dd_payend_pf
(p_id IN dd_pledge.idpledge%TYPE)
RETURN DATE
IS
lv_pay1_dat DATE;
lv_mths_num dd_pledge.paymonths%TYPE;
BEGIN
lv_pay1_dat := dd_paydate1_pf(p_id); --> move it out of SELECT
SELECT paymonths - 1
INTO lv_mths_num
FROM dd_pledge
WHERE idpledge = p_id;
IF lv_mths_num = 0 THEN
RETURN lv_pay1_dat;
ELSE
RETURN ADD_MONTHS(lv_pay1_dat, lv_mths_num);
END IF;
END dd_payend_pf;
See if it helps.

Answer 2

You cannot use the private function as part of a sql query. I've added this answer late as my first answer did not address the issue, and I can see that there is a good answer here now. But just for info and I hope it helps in regard to the DATE usage, here a fun package using a private function (that simply returns the day of the month that you are paid) and a public function that returns your next pay day:
create or replace package date_test is
-- public:
function pay_day return date;
end;
/
create or replace package body date_test is
-- private function:
function paid_on return number is
begin
return 24;
end;
-- public:
function pay_day return date is
-- use private function:
v_paydate_DD number := paid_on;
v_next_payday date;
begin
v_next_payday := trunc(sysdate,'MONTH') + v_paydate_DD - 1;
if trunc(sysdate) >= trunc(v_next_payday) then
select add_months(v_next_payday,1) into v_next_payday from dual;
end if;
return v_next_payday;
end;
end;
/
select date_test.pay_day from dual;