Question

I’m having a problem with a sql query to do the following:

I need to select all products from products table and for each product i need to join a main image (all columns) from images table. Product images table contains image-product relations and a column sortWeight. Main image is the one with the smallest sortWeight.

To get the main product image, i’d do this:

SELECT * FROM images WHERE product=SOME_PRODUCT ORDER BY sortWeight ASC LIMIT 1;

To get all products:

SELECT * FROM products;

Now i need to join these two somehow, but the problem is i don’t know how to bypass a limitation in the following:

SELECT P.* FROM products AS P 
LEFT JOIN 
    (SELECT * FROM images AS I WHERE I.product=P.id ORDER BY sortWeight ASC LIMIT 1) AS I1

The problem is MySQL does not know what P.id is inside the subquery. I’ve also tried it like this:

SELECT P.* FROM products AS P 
LEFT JOIN 
    (SELECT * FROM images AS I WHERE ORDER BY sortWeight ASC LIMIT 1) AS I1 ON (I1.product = P.id)

but i don’t think this gives an accurate result since there’s a limit in subquery and it may select the image from another product with smaller sortWeight.

Can anyone help me rewrite this? Thank you.

Answer 1

Try to join it to subquery that returns images with smallest (min) sortWeight for each product.

This returns smallest sortWeight for each product:

select product, min(sortWeight) as sortWeight
from images
group by product

this is combined version:

SELECT P.*, i.*
FROM products AS P 
left join images as i on p.id = i.product
LEFT JOIN (
    select product, min(sortWeight) as sortWeight
    from images
    group by product
) t on t.product = i.product and i.sortWeight = t.sortWeight