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Databases SQL Basics Finding the Data You Want Review & Practice with SQL Playgrounds

Omar David Laiton
Omar David Laiton
1,005 Points
Posted by Omar David Laiton
Omar David Laiton
1,005 Points

Trying to use LefT JOIN on 3 tables help please

I have 3 tables

"users"

3 columns User-ID int (11) Location varchar (250) Age int (11) Primary-key (User-ID)

"books"

5 columns ISBN varchar (13) Book-title varchar(255) Book-author varchar (255) Year-of-publication int(10) Publisher varchar (255)

"Book-raitings"

3 columns User-ID int (11) ISBN varchar (13) Book-rating int (11) Primary-key (user-id, ISBN)

The Location column varchar will be of format "city, state, country"

I have to generate top 10 books per each country into a separate table. Generate SQL dump.

Thank you for your help in advance.

1 Answer

Steven Parker
Steven Parker
231,184 Points
Steven Parker
Steven Parker
231,184 Points

This is an unusual task because of needing to extract country from Location, and because you want to return the top 10 ranked titles per country. The methods for doing these kinds of things vary between databases, but here's how I might do this using ORACLE:

SELECT title, REGEXP_REPLACE(Location, '.*, ', '') AS Country
FROM books b
LEFT JOIN Book-ratings r on b.ISBN = r.ISBN
LEFT JOIN users u on u.User-ID = r.User-ID
WHERE ROW_NUMBER() OVER
    (PARTITION BY REGEXP_REPLACE(Location,  '.*, ', '') ORDER BY Book-rating) <= 10
ORDER BY Country

In an actual practice, I'd probably modify the users table by breaking Location apart into separate columns to make queries like this a lot easier to perform in the future.