Limit not working in INNER JOINS - mysql

Question

Am sure this question has already been asked, but I can't help but wonder why the "LIMIT 3" in one of the INNER JOINS is only returning a single imgUrl for the specified avp.productID in the "ON clause"! Is that LIMITs do not work in INNER JOINs? What better way could I use to achieve this please?
$query = "SELECT
p.productID,
p.productDesc,
p.productQty,
p.productPr,
p.type,
p.gender,
p.date
From
products AS p
INNER JOIN(
Select
c.productID,
GROUP_CONCAT(DISTINCT c.availCol) AS color_list
FROM
availColors AS c GROUP BY c.productID) AS colors
ON
p.productID = colors.productID
INNER JOIN(
SELECT
s.productID,
GROUP_CONCAT(s.availSizes) AS size_list
FROM
availSizes AS s
GROUP BY
s.productID
) AS sizes
ON
p.productID = sizes.productID
INNER JOIN(
SELECT
avp.productID,
avp.productImg
FROM
availImg AS avp
ORDER BY
avp.productID
LIMIT 3) AS images
ON
images.productID = p.productID
WHERE
p.productID = ?
GROUP BY
p.productID";

Answer 1

Your current query has a sub query that returns 3 images, ordered by product id. This sub query doesn't check if the images belong to the product you are searching for so could easily bring back 3 images for a different product. This would result in the main query trying to do an inner join onto the sub query based on product id and finding no matching records.
As your 2 other sub queries only bring back a single row for each product id (due to the GROUP_CONCAT) and you appear to only want to check one product id at a time then I think you can solve this by just doing a normal join onto the images table (without a sub query) and putting the limit on the main query.
Like this:-
$query = "SELECT
p.productID,
p.productDesc,
p.productQty,
p.productPr,
p.type,
p.gender,
p.date
From products AS p
INNER JOIN
(
Select c.productID,
GROUP_CONCAT(DISTINCT c.availCol) AS color_list
FROM availColors AS c
GROUP BY c.productID
) AS colors
ON p.productID = colors.productID
INNER JOIN
(
SELECT s.productID,
GROUP_CONCAT(s.availSizes) AS size_list
FROM availSizes AS s
GROUP BY s.productID
) AS sizes
ON p.productID = sizes.productID
INNER JOIN availImg images
ON images.productID = p.productID
WHERE p.productID = ?
ORDER BY images.productID, images.productImg
LIMIT 3";
However if you want to bring back multiple product ids then it becomes a bit more complicated. One solution would be to bring back the images with a generated sequence number for each product, and then check in the ON clause of the join that it is one of the first 3 images for a product:-
$query = "SELECT
p.productID,
p.productDesc,
p.productQty,
p.productPr,
p.type,
p.gender,
p.date
From products AS p
INNER JOIN
(
Select c.productID,
GROUP_CONCAT(DISTINCT c.availCol) AS color_list
FROM availColors AS c
GROUP BY c.productID
) AS colors
ON p.productID = colors.productID
INNER JOIN
(
SELECT s.productID,
GROUP_CONCAT(s.availSizes) AS size_list
FROM availSizes AS s
GROUP BY s.productID
) AS sizes
ON p.productID = sizes.productID
INNER JOIN
(
SELECT avp.productID,
avp.productImg,
#seq:=IF(#prev_productID = avp.productID, #seq + 1, 1) AS seq,
#prev_productID := avp.productID
FROM availImg AS avp
CROSS JOIN (SELECT #prev_productID:=0, #seq:=0) sub1
ORDER BY avp.productID
) AS images
ON images.productID = p.productID
AND images.seq <= 3
WHERE p.productID = ?";
Another possible method would be to use GROUP_CONCAT to get a comma separated list of image ids for each product, with SUBSTRING_INDEX to get the first 3 of these, then join that against the images table using FIND_IN_SET to get the rest of the image details:-
$query = "SELECT
p.productID,
p.productDesc,
p.productQty,
p.productPr,
p.type,
p.gender,
p.date
From products AS p
INNER JOIN
(
Select c.productID,
GROUP_CONCAT(DISTINCT c.availCol) AS color_list
FROM availColors AS c
GROUP BY c.productID
) AS colors
ON p.productID = colors.productID
INNER JOIN
(
SELECT s.productID,
GROUP_CONCAT(s.availSizes) AS size_list
FROM availSizes AS s
GROUP BY s.productID
) AS sizes
ON p.productID = sizes.productID
INNER JOIN
(
SELECT avp.productID,
SUBSTRING_INDEX(GROUP_CONCAT(avp.productImg), ',', 2), AS three_images_per_product
FROM availImg AS avp
GROUP BY avp.productID
) AS images
ON images.productID = p.productID
INNER JOIN availImg
ON availImg.productID = images.productID
AND FIND_IN_SET(availImg.productImg, images.three_images_per_product)
WHERE p.productID = ?";
Note that your original query doesn't actually have any image details in the SELECT clause, and also you had done a GROUP BY p.productID which would mean even if 3 images were found the query would only return one row per product id anyway.