MySQL DISTINCT not Filtering out - mysql

Question

I have the folowing sql query:
SELECT DISTINCT(tbl_products.product_id), tbl_products.product_title,
tbl_brands.brand_name, tbl_reviews.review_date_added,
NOW() AS time_now
FROM tbl_products, tbl_reviews, tbl_brands
WHERE tbl_products.product_id = tbl_reviews.product_id AND
tbl_products.brand_id = tbl_brands.brand_id
ORDER BY tbl_reviews.review_date_added DESC
That needs to filter out any duplicate product_id's unfortunatly selecting tbl_reviews.review_date_added makes each record unique which means DISTINCT will not work anymore.
Is there any otherway of doing this query so that product_id is still unique?
I did do the GROUP BY and the problem is I display the tbl_reviews.review_date_added on a website and it selects the oldest date. I need the newest date.
Regards

Answer 1

With the description given, it's a bit hard to be certain, but if review_date_added is the only problem, it seems like you want the MAX() of that date?
If the following doesn't help, please could you give example data, example output, and a description of how you want the output to be created?
SELECT
tbl_products.product_id,
tbl_products.product_title,
tbl_brands.brand_name,
MAX(tbl_reviews.review_date_added) AS review_date_added,
NOW() AS time_now
FROM
tbl_products
INNER JOIN
tbl_reviews
ON tbl_products.product_id = tbl_reviews.product_id
INNER JOIN
tbl_brands
ON tbl_products.brand_id = tbl_brands.brand_id
GROUP BY
tbl_products.product_id,
tbl_products.product_title,
tbl_brands.brand_name
ORDER BY
MAX(tbl_reviews.review_date_added) DESC

Answer 2

Distinct works for the entire row. The parenthesis are just around the field:
distinct (a), b, c === distinct a, b, c
A straightforward solution is group by. You can use min to select the oldest date.
select tbl_products.product_id
, min(tbl_products.product_title)
, min(tbl_brands.brand_name)
, min(tbl_reviews.review_date_added)
, NOW() AS time_now
FROM tbl_products, tbl_reviews, tbl_brands
WHERE tbl_products.product_id = tbl_reviews.product_id AND
tbl_products.brand_id = tbl_brands.brand_id
GROUP BY
tbl_products.product_id
ORDER BY
min(tbl_reviews.review_date_added) DESC
Note that if a product can have multiple brands, this will pick the lowest one.

Answer 3

Try this:
SELECT pr.product_id, pr.product_title,
bd.brand_name,
(SELECT MAX(rev.review_date_added) FROM tbl_reviews rev
WHERE pr.product_id = rev.product_id) AS maxdate,
NOW() AS time_now
FROM tbl_products pr INNER JOIN tbl_reviews re
ON pr.product_id = re.product_id
INNER JOIN tbl_brands bd
ON pr.brand_id = bd.brand_id
GROUP BY pr.product_id
ORDER BY re.review_date_added DESC
or (as suggested by #Hogan)
SELECT pr.product_id, pr.product_title,
bd.brand_name, md.maxdate
NOW() AS time_now
FROM tbl_products pr INNER JOIN tbl_reviews re
ON pr.product_id = re.product_id
INNER JOIN tbl_brands bd
ON pr.brand_id = bd.brand_id
INNER JOIN (SELECT product_id, MAX(review_date_added) AS maxdate
FROM tbl_reviews rev GROUP BY product_id) md
ON pr.product_id = md.product_id
GROUP BY pr.product_id
ORDER BY re.review_date_added DESC

Answer 4

I combined the answer of Andomar with some changes you will find here.
SELECT tbl_products.product_id, tbl_products.product_title,
tbl_products.product_date_added, tbl_brands.brand_name,
MAX(tbl_reviews.review_date_added) AS review_date_added, NOW() AS time_now
FROM tbl_products, tbl_reviews, tbl_brands
WHERE tbl_products.product_id = tbl_reviews.product_id AND
tbl_products.brand_id = tbl_brands.brand_id
GROUP BY tbl_products.product_id
ORDER BY MAX(tbl_reviews.review_date_added) DESC
Works beautifully and shows the newest date at tbl_reviews.review_date_added.
Regards