Writing a SQL query that gets information from 3 different tables - mysql

Question

I cant seem to figure out how to write these 2 queries on the tables that I have created. The two queries that I am trying to write are
Find users that have reviewed both shops and restaurants.
Find users that reviewed businesses, but not shops or restaurants.
The tables that I am using are
reviews;
+-------------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------------+---------+------+-----+---------+-------+
| business_id | int(11) | NO | PRI | NULL | |
| user_id | int(11) | NO | PRI | NULL | |
| review_id | int(11) | NO | PRI | NULL | |
| review_date | date | YES | | NULL | |
| star_rating | int(1) | YES | | 1 |
businesses
+--------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| business_id | int(11) | NO | PRI | NULL | |
| name | varchar(50) | YES | | NULL | |
| city | varchar(40) | YES | | NULL | |
| state | varchar(20) | YES | | NULL | |
| full_address | varchar(120) | YES | | NULL | |
users;
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| user_id | int(11) | NO | PRI | NULL | |
| name | varchar(50) | YES | | NULL | |
| user_since | date | YES | | NULL
explain is_a_restaurant;
+--------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| business_id | int(11) | NO | PRI | NULL | |
| cuisine_type | varchar(20) | YES | | NULL | |
| total_seats | int(11) | YES | | 1 | |
+--------------+-------------+------+-----+---------+-------+
explain is_a_shop;
+-------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| business_id | int(11) | NO | PRI | NULL | |
| shop_type | varchar(50) | YES | | NULL | |
I honestly dont know where to even start. I know I am going to join the businesses with the business ID but I dont know how I am going to find people who reviewed both shops and restaurants. can anyone help?
EDIT: WHat I have tried
For the first query: Find users that have reviewed both shops and restaurants.
SELECT b.business_id
FROM is_a_shop b
JOIN reviews r
ON r.business_id = b.business_id
JOIN is_a_restaurant k
ON r.business_id = k.business_id;

Answer 1

Here's what the first one should look like: you can change the select statement based on what info you want to see.
SELECT u.name
FROM users u
INNER JOIN reviews rev ON rev.user_id = u.user_ID
WHERE EXISTS
(SELECT *
FROM is_a_shop s
WHERE s.business_id = rev.business_id)
OR EXISTS
(SELECT *
FROM is_a_restaurant r
WHERE r.business_id = rev.business_id)
This basically pulls all the users who wrote reviews where the business_id is found in the is_a_shop table or is_a_restaurant. you should be able to figure out the second query easily from this.

Answer 2

Like most things SQL these queries can be solved in a number of different ways. These solutions should be pretty easy to understand:
Find users that have reviewed both shops and restaurants.
-- solution 1 - using joins:
select u.name
from users u
join (
select r.user_id
from reviews r
join is_a_restaurant i on i.business_id = r.business_id
) resturant_reviews on u.user_id = resturant_reviews.user_id
join (
select r.user_id
from reviews r
join is_a_shop i on i.business_id = r.business_id
) shop_reviews on u.user_id = shop_reviews.user_id
-- solution 2: using exists - probably faster than solution 1
select u.name
from users u
join reviews r on u.user_id = r.user_id
where exists (
select 1 from is_a_restaurant i
where i.business_id = r.business_id
) and exists (
select 1 from is_a_shop i
where i.business_id = r.business_id
)
Find users that reviewed businesses, but not shops or restaurants.
-- solution 1: using not in - probably faster than solution 2
select u.name from users u
join reviews r on u.user_id = r.user_id
where r.business_id not in
(
select business_id from is_a_restaurant
union all
select business_id from is_a_shop
)
-- solution 2: using exists
select u.name from users u
join reviews r on u.user_id = r.user_id
where not exists (
select business_id from is_a_restaurant
where r.business_id = business_id
)
and not exists (
select business_id from is_a_shop
where r.business_id = business_id
)