SQL Query involving aggregate functions

Question

I am having issues writing this query. I dont know if I should actually use count because that returns the actual count and I want to return people that havent done a review. Anyway here is my query that I am trying to write.

Find those users that haven’t reviewed any businesses.

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

Here is what I have so far

SELECT reviews.user_id FROM reviews
JOIN businesses ON (reviews.business_id = businesses.business_id)
GROUP BY reviews.user_id ASC
HAVING COUNT(*) > 0;

This returns 0 results and I dont think this is right because someone can be a user and not write a review. But I dont know what else I could. Thanks

EDIT: I figured out that last query but now I am trying to complete this one!

Find the users that have reviewed every business.

Answer 1

You want the users table not the business table.

By left joining the reviews table to the users table we are saying - get me all users and if they haven't left a review just leave those columns as NULLs. then in the where clause, we are selecting only those results where the review columns are nulls, thereby selecting users who haven't left a review.

select u.User_ID 
from 
    users u
    left join reviews r
        on r.user_id = u.user_id
where r.review_id is null

Answer 2

SELECT u.*
FROM users u
WHERE NOT EXISTS ( SELECT 'a'
                   FROM reviews r
                   WHERE r.user_id = u.user_id
                 )