SQL SUM() being multiplied with COUNT() - mysql

Question

dictionaries: id (INT) | title (VARCHAR) | created_at
dictionary_ratings: id (INT) | dictionary_id (INT) | rating (TINYINT)
terms: id (INT) | dictionary_id (INT)
I have three SQL tables as shown above (simplified). I want to query all dictionaries and add information about the number of terms per each, as well as the calculated rating of each dictionary.
SELECT dictionaries.title,
SUM(dictionary_ratings.rating) AS rating_count,
COUNT(terms.id) AS term_count
FROM dictionaries
LEFT JOIN dictionary_ratings
ON dictionaries.id = dictionary_ratings.dictionary_id
LEFT JOIN terms
ON dictionaries.id = terms.dictionary_id
GROUP BY dictionaries.id
ORDER BY dictionaries.created_at DESC
The query above works OK except that it multiplies SUM(dictionary_ratings.rating) by COUNT(terms.id). So if I have 5 terms associated with a dictionary, and the total rating is 10, it outputs 50 instead of 10.
How do I fix that?

Answer 1

You need to modify your query for get your expected result and for that your final query looks like :
SELECT
dictionaries.title,
sub.total_rating AS rating_count,
COUNT(terms.id) AS term_count
FROM
dictionaries
LEFT JOIN (
SELECT
dictionary_id,
SUM(rating) AS total_rating
FROM
dictionary_ratings
GROUP BY
dictionary_id
) sub ON dictionaries.id = sub.dictionary_id
LEFT JOIN terms ON dictionaries.id = terms.dictionary_id
GROUP BY
dictionaries.id
ORDER BY
dictionaries.created_at DESC
Here the first join is between dictionaries table and the result of a
subquery, which calculates the total rating for each dictionary by
summing the ratings for that dictionary in the dictionary_ratings
table and grouping them by dictionary_id.
The second join is between the dictionaries table and the terms table.
And finally, the query then groups the results by the dictionaries.id field and orders
the results by dictionaries.created_at field in desc order.