PL/SQL Function Returning Query Error / ORA-01858: a non-numeric char.....

Question

I'm trying to write a PL/SQL function to dynamically return a query with varying # of columns. I've done this type of thing before wiith much success, however this is the first time I'm using a cursor for loop within a function to accomplish my task.
here is the error I get:
ORA-01858: a non-numeric character was found where a numeric was expected
Error ERR-1101 Unable to process function body returning query.
Mind you I have tested the PL/SQL from a SQL editor and it works fine. I'm confused.
Code is below.
Thanks in advance!
DECLARE
my_query varchar2(4000);
string1 varchar2(50) := 'select city, ';
string2 varchar2(4000) := '' ;
string3 varchar2(4000):= 'from
( select a.title, a.city, a.start_date, sum(a.total_stu) cnt
from sj_class_summary3 a
group by a.title, a.city, a.start_date
having a.title = :P4_COURSE )
group by title, city';
TYPE date_tab_type IS TABLE OF date INDEX BY PLS_INTEGER;
date_tbl date_tab_type;
i number;
BEGIN
i:=1;
for myrec in (select distinct start_date from sj_class_summary3
where start_date between :P4_START_DATE and :P4_END_DATE order by 1) loop
date_tbl(i) := myrec.start_date;
string2 := string2 || ' max( decode( start_date, ''' || date_tbl(i) || ''', cnt,0) ) "' || date_tbl(i) || '", ';
i := i+ 1;
end loop;
string2 := SUBSTR(string2,1,LENGTH(string2)-2);
string2 := string2 || ' ';
my_query := string1 || string2 || string3;
return my_query;
END;

Answer 1

Hi Bob,
you also have another date to character to date conversion in:
decode( start_date, ''' || date_tbl(i) || ''', cnt,0)
does this need to be:
decode( start_date, to_date(''' || to_char(date_tbl(i), 'dd/mm/yyyy') || ''', ''dd/mm/yyyy''), cnt,0)
Regards
Michael