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Editorial Team
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Asked: 2026-06-14T20:16:16+00:00

Here’s my database: This query select all supplements: SELECT a.id, a.name, a.image, a.url_segment, COUNT(b.id)

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Here’s my database:

enter image description here

This query select all supplements:

SELECT a.id, a.name, a.image, a.url_segment, COUNT(b.id) AS reviews_count, ROUND(AVG(b.rating), 2) AS reviews_rating, (((SELECT COUNT(*) FROM reviews) * (SELECT AVG(rating) FROM reviews)) + (COUNT(b.id) * AVG(b.rating))) / ((SELECT COUNT(*) FROM reviews) + COUNT(b.id)) AS bayesian_rating
FROM (`supplements` AS a)
LEFT JOIN `reviews` AS b ON `b`.`supplements_id` = `a`.`id`
GROUP BY `a`.`id`
ORDER BY `bayesian_rating` DESC

And this, all supplements from one subcategory (in this case, with id = 1): 

SELECT a.id, a.name, a.image, a.url_segment, COUNT(b.id) AS reviews_count, ROUND(AVG(b.rating), 2) AS reviews_rating, (SELECT text FROM reviews WHERE supplements_id = a.id ORDER BY id DESC LIMIT 1) AS reviews_latest_text, (((SELECT COUNT(*) FROM reviews LEFT JOIN supplements ON (supplements.id = reviews.supplements_id AND supplements.subcategories_id = 1)) * (SELECT AVG(rating) FROM reviews LEFT JOIN supplements ON (supplements.id = reviews.supplements_id AND supplements.subcategories_id = 1))) + (COUNT(b.id) * AVG(b.rating))) / ((SELECT COUNT(*) FROM reviews LEFT JOIN supplements ON (supplements.id = reviews.supplements_id AND supplements.subcategories_id = 1)) + COUNT(b.id)) AS bayesian_rating
FROM (`supplements` AS a)
LEFT JOIN `reviews` AS b ON `b`.`supplements_id` = `a`.`id`
WHERE `a`.`subcategories_id` =  '1'
GROUP BY `a`.`id`
ORDER BY `bayesian_rating` DESC

The bayesian rating of the same supplements should be different on each query, but on both it’s returning the same.

Here’s is the part where I calculate the bayesian rating on the first query:

(((SELECT COUNT(*) FROM reviews) * (SELECT AVG(rating) FROM reviews)) + (COUNT(b.id) * AVG(b.rating))) / ((SELECT COUNT(*) FROM reviews) + COUNT(b.id)) AS bayesian_rating

On the second:

(((SELECT COUNT(*) FROM reviews LEFT JOIN supplements ON (supplements.id = reviews.supplements_id AND supplements.subcategories_id = 1)) * (SELECT AVG(rating) FROM reviews LEFT JOIN supplements ON (supplements.id = reviews.supplements_id AND supplements.subcategories_id = 1))) + (COUNT(b.id) * AVG(b.rating))) / ((SELECT COUNT(*) FROM reviews LEFT JOIN supplements ON (supplements.id = reviews.supplements_id AND supplements.subcategories_id = 1)) + COUNT(b.id)) AS bayesian_rating

For some reason the join is not making any difference to the result.

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1 Answer

  1. Editorial Team

    Since reviews are children of supplements, joining to supplements won’t return anything different for reviews.

    I think it’s expected that the two queries return the same.

    Also, by mathematical definition of AVG, the term

    (SELECT COUNT(*) FROM reviews) * (SELECT AVG(rating) FROM reviews)

    is identical to

    (SELECT SUM(rating) FROM reviews)

    so you can simplify (and speed up) your query but substituting this in.

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