How to avoid no data exception? - database

Question

I am working on a project and I have a database schema written in Oracle db for a library. I have some issues when trying to build a function which should provide some recommendations for the user.
The recommendations should be given based on the last books genres some user had borrowed in the past month and the most read books of all users. Every time I try to run the function I get a no data error even if I have values in the tables.
CREATE OR REPLACE FUNCTION getGenre(bookISBN IN VARCHAR2) RETURN VARCHAR2
AS
v_bookGenre VARCHAR2(100);
BEGIN
select genre into v_bookGenre from (select genre from books where bookISBN=isbn);
return v_bookGenre;
END getGenre;
/
CREATE OR REPLACE TYPE t IS TABLE OF varchar2(200);
/
create or replace type typeRecom is VARRAY(200) of VARCHAR2(200);
/
CREATE OR REPLACE FUNCTION topRecommandations(idUser IN VARCHAR2)
RETURN typeRecom
AS
lastGenres t :=t();
readedBooks t:=t();
topTen typeRecom;
v_i NUMBER := 1;
v_j NUMBER := 1;
verifDateBook NUMBER:=0;
verifDateLoan NUMBER:=0;
BEGIN
select count(*) into verifDateBook from books;
if(verifDateBook = 0) then
raise no_data_found;
end if;
select count(*) into verifDateLoan from loans;
if(verifDateLoan = 0) then
raise no_data_found;
end if;
lastGenres.extend();
for i in (select genre from books join loans on books.isbn=loans.bookId where loanDate>add_months(sysdate,-1) and idUser=loans.regNo order by genre desc) loop
DBMS_OUTPUT.PUT_LINE(i.genre);
lastGenres.extend();
lastGenres(lastGenres.count) := i.genre;
end loop;
/*select title into bookTitles from book join loan on book.isbn=loan.regNo;*/
readedBooks.extend();
for i in (select bookId from loans group by bookId order by count(bookId) desc) loop
readedBooks.extend();
readedBooks(readedBooks.count) := i.bookId;
end loop;
--select bookId into readedBooks from loan order by count(bookId) desc;
for v_i IN 1..50 LOOP
for v_j in 1..50 LOOP
DBMS_OUTPUT.PUT_LINE(lastGenres(v_j));
DBMS_OUTPUT.PUT_LINE(getGenre(readedBooks(v_i)));
if(lastGenres(v_j)=getGenre(readedBooks(v_i))) then
topTen(v_i) := getGenre(readedBooks(v_j));
end if;
EXIT WHEN topTen.count=10;
END LOOP;
EXIT WHEN topTen.count=10;
end LOOP;
for v_cont in 1..10 LOOP
--if(topTen.count <= 10) then
DBMS_OUTPUT.PUT_LINE(topTen(v_cont));
--end if;
end loop;
return topTen;
EXCEPTION
WHEN no_data_found THEN
DBMS_OUTPUT.PUT_LINE('Nu exista valori.');
END topRecommandations;
Expected result: a list with 10 recommended books.
Actual result: no data found error.

Answer 1

Unless I'm wrong, the only place that might return NO_DATA_FOUND is the getGenre function, as it has a SELECT some value into a variable, and no exceptions are handled. Other SELECTs use aggregates (which will return 0 if there's nothing there).
Therefore, fix that function, e.g.
CREATE OR REPLACE FUNCTION getGenre(bookISBN IN VARCHAR2)
RETURN VARCHAR2
AS
v_bookGenre VARCHAR2(100);
BEGIN
select genre
into v_bookGenre
from books where bookISBN = isbn;
return v_bookGenre;
EXCEPTION
when no_data_found then
return null;
END getGenre;
By the way, why did you use an inline view in that function? Why didn't you simply return genre from the table itself (like I did)?