SQL Query with multiple Inner Joins returns wrong Count of Values - sql

Question

I'm new to sql and I'm practicing on a created database in oracle about airbnb listings in amsterdam and berlin. I'm trying to JOIN the Tables HOSTS (host_id, host_name), LISTINGS (which includes all listings in the two city with the attributes listings_id, listings_name, Price, host_id...), Neighbourhoods (Neighbourhood_Group, City and Neighbourhood) and reviews (with review_id, listings_id as a foreign key, reviewer_id, reviewer_name and comment).
Now i want to write a query, which returns the average price, lowest price, highest price, the city (Berlin or Amsterdam), the neighbourhood (Centrum, Alexanderplatz...), the amount of listings and the amount of reviews, all grouped by the distinct neighbourhood, and by the WHERE statement, that limits the returns to those listings, that are hosted by Hosts which have less than 3 listings total.
Now if I run the query without the reviews table and only order it by amount_listings, I get the correct amount of listings per neighbourhood for the column "amount_listings".
SELECT avg(l.price) as Mean_Price,
n.city,
n.neighbourhood,
count (l.listings_id) as amount_listings,
min(l.price),
max(l.price)
FROM listings l
INNER JOIN neighborhood n
ON l.neighbourhood = n.neighbourhood
INNER JOIN hosts h
ON l.host_id = h.host_id
WHERE h.host_id IN (
SELECT host_id
FROM listings
GROUP BY host_id
HAVING count(host_id) < 3
)
GROUP BY n.neighbourhood, n.city
ORDER BY amount_listings DESC;
But if i include the amount of reviews in the query, the results are incorrect. The amount of reviews and the amount of listings column show the wrong amount (too much).
SELECT avg(l.price) as Mean_Price, count(l.listings_id) as amount_listings,
min(l.price), max(l.price), n.city, n.neighbourhood, count(r.review_id) as amount_reviews
FROM listings l
INNER JOIN neighborhood n
ON l.neighbourhood = n.neighbourhood
INNER JOIN hosts h
ON l.host_id = h.host_id
INNER JOIN reviews r
ON l.listings_id= r.listings_id
WHERE h.host_id IN (
SELECT host_id
FROM listings
GROUP BY host_id
HAVING count(host_id) < 3
)
GROUP BY n.neighbourhood, n.city
ORDER BY amount_listings DESC, amount_reviews DESC;
I dont know why the amount_listings and amount_reviews return such wrong results.

Answer 1

Aggregate before joining. You want to join the review count to each listing and then the aggregated listing information to the neighbourhood.
select
lr.mean_price,
n.city,
n.neighbourhood,
lr.amount_listings,
lr.min_price,
lr.max_price,
lr.amount_reviews
from neighborhood n
join
(
select
l.neighbourhood,
min(l.price) as min_price,
max(l.price) as max_price,
avg(l.price) as mean_price,
count(*) as amount_listings,
coalesce(sum(r.reviews_for_listing), 0) as amount_reviews
from listings l
left join
(
select
listings_id,
count(*) as reviews_for_listing
from reviews
group by listings_id
) r on r.listings_id = l.listings_id
where l.host_id in
(
select host_id
from listings
group by host_id
having count(*) < 3
)
group by l.neighbourhood
) lr on lr.neighbourhood = n.neighbourhood
order by n.city, n.neighbourhood;

Answer 2

The cause is that since you are adding a new table with more than 0 rows, records are counted several times, depending on how many reviews are. Since you are only interested in the count of reviews, let's join that instead with a nice old trick:
SELECT avg(l.price) as Mean_Price, count(l.listings_id) as amount_listings,
min(l.price), max(l.price), n.city, n.neighbourhood, r.review_count as amount_reviews
FROM listings l
INNER JOIN neighborhood n
ON l.neighbourhood = n.neighbourhood
INNER JOIN hosts h
ON l.host_id = h.host_id
INNER JOIN (select reviews.listings_id as listings_id, count(*) as review_count from reviews where reviews.listings_id = l.listings_id) r
ON l.listings_id= r.listings_id
WHERE h.host_id IN (
SELECT host_id
FROM listings
GROUP BY host_id
HAVING count(host_id) < 3
)
GROUP BY n.neighbourhood, n.city
ORDER BY amount_listings DESC, amount_reviews DESC;