MYSQL-连接或子查询问题

Question

I am trying to get a query to return all the relevant records for products which feature in my featured projects table. All is working for the simple MYSQL query however I want to get the Thumb Image for each product two which is stored in another table (product_image) but with a catch. In this "product_image" table not every products (product_id) will have a record as it is only populated on upload of images and some products may have several records (images stored) I only want one record returned for each product ID. So a product record in the products table has a 0 to many relationship with the product_image table.

I have a working basic SQL statement for the basics without returning the image, I can code a query that only returns the products who have records in the product image table but I need the query to return all the rows in the outer query and then 1 thumb_img that matches the product id from the image table and if there is no images it can return a Null value.

Is this possible? Below is my latest attempt but this just returns the matching thumb for the first product_ID and duplicates this for the other 3(in this case due to limit at end) records. I need the matching thumb for each record rather then the matching one for the first record repeated between the rest. And if there is no records for that product ID a null returned.

SELECT * FROM `products`, `featured_products`, `shop`, (SELECT thumbSrc
          FROM product_image
         WHERE products.productId = product_image.productID
        ORDER BY product_image.position ASC
         LIMIT 1) image
where shop.shopId = products.shopId AND featured_products.productId = products.productId AND visible = '1' LIMIT 4

Thanks in advance for any help / feedback. Is it possible to do this as I want to keep my database as normalized as possible rather then result in storing a thumb field in the products table too.

Answer 1

Untested, and there may be a better (performance) approach to the subselect... on PI I'm doing.

Effectively what this is tying to do is return all products, the related feature products, and shops. Then return only those images which have a matching product; but only if the image is the product image with the lowest position. This way the product, feature_product and shops don't get excluded if an image is missing (thus the nature of a LEFT [outer] join)

SELECT *
FROM PRODUCTS P
INNER JOIN Featured_Products FP
 on P.ProductID = FP.ProductID
INNER JOIN SHOP S
 P.ShoPID = S.ShopID
LEFT JOIN (Select thmbsrc, productID, position, min(Position) as MinPosition
           FROM Product_Image PI
           group by thmbsrc, productID, Position
           having MinPosition = position) PI
 on PI.ProductID = P.ProductID

Performance may be improved if we first get a list of all the productIDs with the lowest position and then join it back to the product_image to get imgsrc. Something like ( Select min(position) minPos, productID from product_Image ) may allow the SQL engine to look at each record once, and not for every value in a having clause (if position is indexed then this would even be faster).

So...

SELECT *
FROM PRODUCTS P
INNER JOIN Featured_Products FP
 on P.ProductID = FP.ProductID
INNER JOIN SHOP S
 P.ShoPID = S.ShopID
LEFT JOIN (Select imgsrc FROM Product_Image PI
           INNER JOIN (Select min(position) minPos, productID 
                       from product_Image
                       GROUP BY ProductID) PI2
            on PI.ProductID = PI2.ProductID
           and PI.position = PI2.PminPos)
 on PI.ProductID = P.ProductID

May perform better depending on table statistics and available indexes.