What order (Invoice) was the most expensive? Which one was the cheapest?

SELECT MAX(total)
FROM Invoice;

SELECT MIN(total)
FROM Invoice;

Which city (BillingCity) has the most orders?

SELECT Invoice.BillingCity, SUM(Invoice.Total)
FROM Invoice
GROUP BY Invoice.BillingCity
ORDER BY (Invoice.Total) DESC;

Calculate (or count) how many tracks have this MediaType: Protected AAC audio file.

SELECT COUNT (Track.MediaTypeId), MediaType.Name
FROM Track
INNER JOIN MediaType ON Track.MediaTypeId=MediaType.MediaTypeId
WHERE MediaType.Name='Protected AAC audio file';

Find out what Artist has the most albums? (hint: check ORDER BY)

SELECT Artist.Name, COUNT(Artist.ArtistId)
FROM Artist
INNER JOIN Album ON Artist.ArtistId=Album.ArtistId
GROUP BY Artist.ArtistId HAVING COUNT(Artist.ArtistId)
ORDER BY COUNT(Artist.ArtistId) DESC;

----otra solución----

SELECT Artist.Name, COUNT(Artist.ArtistId)
FROM Album
INNER JOIN Artist ON Album.ArtistId=Artist.ArtistId
GROUP BY Album.ArtistId
ORDER BY COUNT(Artist.ArtistId) DESC;

What genre has the most tracks?

SELECT Genre.Name, COUNT(Track.GenreId)
FROM Track
INNER JOIN Genre ON Track.GenreId=Genre.GenreId
GROUP BY Track.GenreId HAVING COUNT(Track.GenreId)
ORDER BY COUNT(Track.GenreId) DESC;

----otra solución----
SELECT Genre.Name, COUNT(Track.GenreId)
FROM Genre
INNER JOIN Track ON Track.GenreId=Genre.GenreId
GROUP BY Track.GenreId
ORDER BY COUNT(Track.GenreId) DESC;

Which customer spent the most money so far?

SELECT Customer.FirstName, Customer.LastName, Customer.CustomerId, SUM(Invoice.Total)
FROM Invoice
INNER JOIN Customer ON Invoice.CustomerId=Customer.CustomerId
GROUP BY Customer.CustomerId HAVING SUM(Invoice.Total)
ORDER BY SUM(Invoice.Total)DESC;

----otra solución----
SELECT Customer.FirstName, Customer.LastName, Customer.CustomerId, SUM(Invoice.Total)
FROM Customer
INNER JOIN Invoice ON Invoice.CustomerId=Customer.CustomerId
GROUP BY Customer.CustomerId
ORDER BY SUM(Invoice.Total)DESC;

What songs were bought with each order?

SELECT Invoice.InvoiceId, InvoiceLine.TrackId, Track.Name
FROM Invoice
INNER JOIN InvoiceLine ON Invoice.InvoiceId=InvoiceLine.InvoiceId
INNER JOIN Track ON InvoiceLine.TrackId=Track.TrackId;