Cursor loop - COUNT from two tables - PLSQL Oracle - oracle

Question

Need some advice here. I have two tables: 1) Car owners; 2) Cars. With cursor loop I need to count how many cars owns each person. I don't understand how do you write the select code when two tables are involved.
CREATE TABLE car_owner(
pnr VARCHAR2(13) PRIMARY KEY,
fname VARCHAR2(20);
CREATE TABLE car(
regnr VARCHAR2(6) PRIMARY KEY,
pnr REFERENCES car_owner(pnr);
INSERT INTO car_owner VALUES('19490321','anna');
INSERT INTO car_owner VALUES('19540201','tomas');
INSERT INTO car_owner VALUES('19650823','roger');
INSERT INTO car VALUES('ase456','19490321');
INSERT INTO car VALUES('ptg889','19490321');
INSERT INTO car VALUES('bon666','19650823');
declare
cursor c_cars is select pnr, fnamn, count(amount_of_cars(pnr))
from car_owner, car;
v_pnr car_owner.pnr%type;
v_fnamn car_owner.fnamn%type;
begin
if not c_cars %isopen then
open c_cars;
end if;
loop
fetch c_cars
into v_pnr, v_fnamn;
exit when c_cars %notfound;
dbms_output.put_line(v_pnr || ', ' || v_fnamn || ', ' ||
', owns: ' || amount_of_cars || ' cars');
end loop;
close c_cars;
end;
So the answer should look like this:
19490321, anna, owns: 2 cars
19540201, tomas, owns: 1 car
19650823, roger, owns: 0 cars

Answer 1

Join those tables. As you're aggregating something, all non-aggregated columns should be part of the group by clause.
Also, consider using a cursor FOR loop as it is way simpler (Oracle does all the dirty job for you - you don't have to declare variable(s), open the cursor, worry about exiting the loop, close the cursor).
SQL> DECLARE
2 CURSOR c_cars IS
3 SELECT o.pnr, o.fname, COUNT (*) cnt
4 FROM car_owner o JOIN car c ON c.pnr = o.pnr
5 GROUP BY o.pnr, o.fname;
6 BEGIN
7 FOR cur_r IN ( SELECT o.pnr, o.fname, COUNT (*) cnt
8 FROM car_owner o JOIN car c ON c.pnr = o.pnr
9 GROUP BY o.pnr, o.fname)
10 LOOP
11 DBMS_OUTPUT.put_line (
12 cur_r.pnr
13 || ', '
14 || cur_r.fname
15 || ', '
16 || ', owns: '
17 || cur_r.cnt
18 || ' car(s)');
19 END LOOP;
20 END;
21 /
19650823, roger, , owns: 1 car(s)
19490321, anna, , owns: 2 car(s)
PL/SQL procedure successfully completed.
SQL>