MySQL JOIN, GROUP BY, ORDER BY

Question

I have a table of products:

CREATE TABLE products (`id` INT);

And a table of images for those products:

CREATE TABLE images (`id` INT, `product_id` INT, `default` TINYINT(1));

I need to select all the products, and join the images table so that images with (default = 1) will be preferred, and if a product has no images with (default = 1), an image with (default = 0) will be shown in its place.

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Here's an image showing what I'm looking for:

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Right now I have this query:

SELECT p.id, i.id
FROM products AS p
LEFT JOIN (
    SELECT product_id, url
    FROM images
    ORDER BY default
) AS i
ON p.id = i.product_id
GROUP BY p.id
ORDER BY p.name

Which doesn't prioritize "default" images. The subquery doesn't seem to do anything.

Answer 1

SQLFiddle demo

select products.id,
       coalesce(t1.mid,t2.mid) as image_id      

from products
left join (select min(id) mid,product_id 
                  from images where `default`=1
                  group by product_id ) t1
        on products.id=t1.product_id
left join (select min(id) mid,product_id 
                  from images where `default`=0
                  group by product_id ) t2
        on products.id=t2.product_id

Answer 2

Try this:

SELECT 
  p.id productid, 
  IFNULL(i1.id, i2.id)
FROM products    AS p
LEFT JOIN images AS i1  ON p.id = i1.product_id
                       AND i1.`Default` = 1  
LEFT JOIN images AS i2  ON p.id = i2.product_id
                       AND i2.`Default` = 0
GROUP BY p.id;

Answer 3

If you want to show only one image for product, then try this query -

SELECT p.*, i.* FROM products p
  JOIN (SELECT * FROM images ORDER BY `default` DESC) i
    ON p.id = i.product_id
GROUP BY p.id

Answer 4

Looks like I was just missing a 'DESC' in the subquery's ORDER BY

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