Multi cursor in oracle function - oracle

Question

I want to create an oracle function that get the user_id as param and return varchar2 that contain the e-mail text for the books that the user take from the library and didn't return them back, my email text that I want is:
"hello "
You should return this books:
1 <BOOK_NAME> that taken at
2. ....
This is what I wrote so far
create or replace function get_un_recived_books(param_client_id in number) return varchar2 is
Result varchar2(2000);
cursor cur_book is
select * from hashala natural join client natural join all_books natural join book
where
recived =0
and recived_date is null
and clientid= param_client_id
and taken_date < add_months(trunc(sysdate, 'month'), -3);
begin
FOR b IN cur_book LOOP
Result:= 'book ' || b.book_name;
END LOOP;
return(Result);
end;
I have 3 problems
how can i return all the books and not the last one (like += in c)
for the client name do I need to add another cursor, can I do it?
how to pass the value from the original query
select get_un_recived_books(!!!ADD HERE THE CLIENID!!!), clientid from hashala natural join client natural join all_books natural join
book
where
recived =0
and recived_date is null
and taken_date < add_months(trunc(sysdate, 'month'), -3);

Answer 1

I don't have your tables so I'll use Scott's sample schema to illustrate how you might do that. Read comments within code.
Function:
SQL> create or replace function f_test (par_deptno in number)
2 return varchar2
3 is
4 l_dname dept.dname%type;
5 retval varchar2(4000);
6 begin
7 -- department name (in your case, client name)
8 select dname
9 into l_dname
10 from dept
11 where deptno = par_deptno;
12
13 -- loop through employees in PAR_DEPTNO (in your case,
14 -- books client borrowed)
15 for cur_r in (select ename, hiredate
16 from emp
17 where deptno = par_deptno
18 )
19 loop
20 -- this is what you're missing: "retval := retval || ..."
21 retval := retval || 'Name: ' || cur_r.ename ||
22 ', borrowed on ' || to_char(cur_r.hiredate, 'dd.mm.yyyy') || chr(10);
23 end loop;
24
25 retval := 'Hello, ' || l_dname ||chr(10) ||
26 'you borrowed the following books and didn''t return them yet.' || chr(10) ||
27 retval;
28 return retval;
29 end;
30 /
Function created.
Testing:
SQL> select f_test(10) from dual;
F_TEST(10)
--------------------------------------------------------------------------------
Hello, ACCOUNTING
you borrowed the following books and didn't return them yet.
Name: CLARK, borrowed on 09.06.1981
Name: KING, borrowed on 17.11.1981
Name: MILLER, borrowed on 23.01.1982
SQL>
If you - as you said - want to pass the ID "dynamically", just include it into the function. Something like this (I'm retrieving data for departments 10 and 30, for everyone who works as a clerk):
SQL> select f_test(d.deptno)
2 from dept d join emp e on e.deptno = d.deptno
3 where d.deptno in (10, 30)
4 and e.job = 'CLERK';
F_TEST(D.DEPTNO)
------------------------------------------------------------------------
Hello, SALES
you borrowed the following books and didn't return them yet.
Name: ALLEN, borrowed on 20.02.1981
Name: WARD, borrowed on 22.02.1981
Name: MARTIN, borrowed on 28.09.1981
Name: BLAKE, borrowed on 01.05.1981
Name: TURNER, borrowed on 08.09.1981
Name: JAMES, borrowed on 03.12.1981
Hello, ACCOUNTING
you borrowed the following books and didn't return them yet.
Name: CLARK, borrowed on 09.06.1981
Name: KING, borrowed on 17.11.1981
Name: MILLER, borrowed on 23.01.1982
SQL>

Answer 2

You may use LISTAGG function to achieve this. This task can be solved in pure SQL via view, which will not produce child cursors compared to function invocation. You just need to filter it if you wish to select individual clients.
However you can also put the same text in a function if you wish, which also doesn't require any loops. Because from Oracle 12C and above you may extend varchar2 up to 32767 bytes with MAX_STRING_SIZE = EXTENDED parameter, which removes the boundary of varchar2 in SQL compared to PL/SQL.
Both the examples are below:
create table client
as
select
level as id
, 'Client ' || level as name
from dual
connect by level < 10
create table book
as
select
level as id
, 'Book ' || level as name
from dual
connect by level < 100
create table borrowed_book
as
select
trunc(level / 4) + 1 as client_id
, level as book_id
, add_months(sysdate, -mod(level, 11)) as dt
, case mod(level, 4) when 1 then 1 else 0 end as received
from dual
connect by level < 60
create view v_reminder
as
select
bb.client_id,
c.name as client_name,
'Hello, ' || max(c.name) || '!'
|| chr(10) || chr(10)
|| 'You should return this books:' || chr(10)
|| listagg(b.name || ' borrowed on ' || to_char(bb.dt, 'dd-Mon-yyyy'), chr(10))
within group(order by bb.dt desc, bb.book_id) as rem
from borrowed_book bb
join client c
on bb.client_id = c.id
join book b
on bb.book_id = b.id
where bb.received = 0
and bb.dt < add_months(sysdate, -3)
group by bb.client_id, c.name
create or replace function f_get_reminder(p_client_id in int, p_dt_offset in int default 3)
return varchar2
as
res varchar2(32000);
begin
select
'Hello, ' || max(c.name) || '!'
|| chr(10) || chr(10)
|| 'You should return this books:' || chr(10)
|| listagg(b.name || ' borrowed on ' || to_char(bb.dt, 'dd-Mon-yyyy'), chr(10))
within group(order by bb.dt desc, bb.book_id)
into res
from borrowed_book bb
join client c
on bb.client_id = c.id
join book b
on bb.book_id = b.id
where bb.received = 0
and bb.dt < add_months(sysdate, -p_dt_offset)
and bb.client_id = p_client_id
/*Added this line to get NULL as output, because aggregate function
without group by always returns a row even for empty input dataset*/
group by bb.client_id
;
return res;
end;
/
select
c.id
, c.name
, f_get_reminder(c.id) as rem
from client c
ID
NAME
REM
1
Client 1
null
2
Client 2
Hello, Client 2!You should return this books:Book 4 borrowed on 27-Feb-2021Book 6 borrowed on 27-Dec-2020Book 7 borrowed on 27-Nov-2020
3
Client 3
Hello, Client 3!You should return this books:Book 8 borrowed on 27-Oct-2020Book 10 borrowed on 27-Aug-2020
4
Client 4
Hello, Client 4!You should return this books:Book 15 borrowed on 27-Feb-2021
5
Client 5
Hello, Client 5!You should return this books:Book 16 borrowed on 27-Jan-2021Book 18 borrowed on 27-Nov-2020Book 19 borrowed on 27-Oct-2020
6
Client 6
Hello, Client 6!You should return this books:Book 20 borrowed on 27-Sep-2020
7
Client 7
Hello, Client 7!You should return this books:Book 26 borrowed on 27-Feb-2021Book 27 borrowed on 27-Jan-2021
8
Client 8
Hello, Client 8!You should return this books:Book 28 borrowed on 27-Dec-2020Book 30 borrowed on 27-Oct-2020Book 31 borrowed on 27-Sep-2020
9
Client 9
Hello, Client 9!You should return this books:Book 32 borrowed on 27-Aug-2020
db<>fiddle here