1. OUTER JOIN

SELECT * 
FROM dtaniar.book5 b, dtaniar.review5 r
WHERE b.ISBN=r.ISBN(+);

SELECT * 
FROM dtaniar.book5 b 
 LEFT JOIN dtaniar.review5 r
 ON b.ISBN=r.ISBN;

2. NVL()

This is the syntax to set the null value to 0:
NVL (attribute_name, 0)

3. ROUND(attribute_name)

When creating the TempBookWithAvgStar, the average of stars for each book 
and category should be either 0, or 1,2,3,4,5. Thus, we need to use ROUND to 
make sure the average value is an integer.

This is the syntax to set a number into an integer:

ROUND(attribute_name)

4. ROWNUM

select * from (
 select 
 c.CATEGORYID, 
 c.CATEGORYDESCRIPTION, 
 sum(f.num_of_books) as total_num_books
 from booksalesfact f, categorydim c
 where f.categoryid = c.categoryid
 group by c.CATEGORYID, c.CATEGORYDESCRIPTION
 order by total_num_books desc)
where rownum =1;