What is the average rating of all items?
select avg(rating) from review
How many items have a rating lower than the average?
select count(*) from item as t1 join review as t2 on t1.i_id = t2.i_id where t2.rating < (select avg(rating) from review)
How about higher than the average?
select count(*) from item as t1 join review as t2 on t1.i_id = t2.i_id where t2.rating > (select avg(rating) from review)
What are the names of those items?
select t1.title from item as t1 join review as t2 on t1.i_id = t2.i_id where t2.rating > (select avg(rating) from review)