help me wite query for these question

a. Find all those customers who have not purchased anything from Niles Video Inc.

SELECT CustomerID,Name FROM CustomerINfo CI
where customerID not exists(
select customerID from OrderQuery oq inner join Movies m on m.itemNumber=oq.itemnumber
where Genre='Niles Video Inc')

b. Get the total number of DVDs and video tapes sold per genre.

> select genre,sum(quantity) from
> OrderQuery oq inner join Movies m on
> m.itemnumber=oq.itemnumber group by
> m.genre

c. Get the average number of DVDs per purchase.

select Title,avg(sum(quantity)) from Movies m inner join OrderQuery oq on oq.itemnumber=m.itemnumber
group by Title