Question

Given this sample dataset:

item
----
item_id  item_name  item_added
1        Apple      <date_time>
2        Banana     <date_time>

user
----
user_id  user_name
1        Alice
2        Bob
3        Carol

rating
------
rating_id  item_id  user_id  rating_value
1          1        1        3
2          1        2        4
3          1        3        5
4          2        1        5
5          2        2        2

I have this query to figure out which items are missing only one rating:

SELECT item.*,
       COUNT(rating_value) AS rating_count
  FROM item
LEFT JOIN rating ON item.item_id = rating.item_id
 GROUP BY item_id
   HAVING rating_count = 2
 ORDER BY item_name

How can I modify this query to show items that are only missing Carol’s rating? The rating_count row is not essential. The only essential columns are item_id and item_name.

Thank you.

Answer 1

This will output the items only missing Carol’s rating (user_id = 3 that needs to be passed as a parameter):

SELECT item.*, COUNT(rating.rating_value) AS rating_count 
 FROM item INNER JOIN rating ON item.item_id = rating.item_id AND 
      (3 NOT IN (SELECT r.user_id FROM rating AS r 
          WHERE r.item_id = rating.item_id))
 GROUP BY item_id
 HAVING rating_count = 2   /* (2 = number of users - 1) */
 ORDER BY item_name;

You have to replace all item by movie but your query mixes item and movie and the table was item so I did the query with item and I only saw the movie in the query too late.