MySQL JOIN、GROUP BY、ORDER BY

Question

我有一张产品表：

CREATE TABLE products (`id` INT);

以及这些产品的图片表：

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

我需要选择所有产品，并加入图像表，以便首选带有 (default = 1) 的图像，如果产品没有带有 (default = 1) 的图像，则使用带有 (default = 1) 的图像(default = 0) 将显示在其位置。

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这是一张显示我正在寻找的图片：

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现在我有这个查询：

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

不优先考虑“默认”图像。子查询似乎没有做任何事情。

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

看起来我只是在子查询的 ORDER BY 中遗漏了一个“DESC”

:\

Answer 3

如果您只想为产品显示一张图片，请尝试此查询 -

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

试试这个：

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;