MySQL problem with statement

问题

SELECT a.id,i.* FROM ads a
    INNER JOIN images i ON i.ad_id=a.id
        GROUP BY a.id
            LIMIT 10

Can't figure out, how to pick images with flag "main" inside images table.

For one a.id can be up to 3 photos inside images table, one of those 3 photos can have a field main=1. I need to pick photos by priority where flag main is set to 1.

CREATE DATABASE IF NOT EXISTS `test123`;
USE `test123`;

CREATE TABLE IF NOT EXISTS `ads` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=229 DEFAULT CHARSET=utf8;

INSERT INTO `ads` (`id`) VALUES (217), (225), (226), (228);

CREATE TABLE IF NOT EXISTS `images` (
  `image_id` int(10) NOT NULL AUTO_INCREMENT,
  `ad_id` int(10) DEFAULT '0',
  `main` int(10) DEFAULT '0',
  `t_0` varchar(255) DEFAULT '0',
  `t_1` varchar(255) DEFAULT '0',
  `t_8` varchar(255) DEFAULT '0',
  PRIMARY KEY (`image_id`),
  KEY `ad_id` (`ad_id`),
  KEY `t_0` (`t_0`),
  KEY `t_1` (`t_1`),
  KEY `t_8` (`t_8`),
  KEY `main` (`main`)
) ENGINE=MyISAM AUTO_INCREMENT=425 DEFAULT CHARSET=utf8;


INSERT INTO `images` (`image_id`, `ad_id`, `main`, `t_0`, `t_1`, `t_8`) VALUES 
(1, 226, 0, 'img_link1', 'img_link2', 'img_link3'),
(2, 228, 0, 'img_link1', 'img_link2', 'img_link3'),
(3, 225, 0, 'img_link1', 'img_link2', 'img_link3'),
(4, 217, 0, 'img_link1', 'img_link2', 'img_link3'),
(5, 217, 1, 'img_link1', 'img_link2', 'img_link3'),
(6, 217, 0, 'img_link1', 'img_link2', 'img_link3');

Something like that but only 1 row per ads.id

SELECT a.id, i.main AS main
    FROM images i
        LEFT JOIN ads a ON a.id=i.ad_id
            WHERE i.main=1 OR i.main=0

Output required:

id  | main | links
217 | 1    | ...
225 | 0    | ...
226 | 0    | ...
228 | 0    | ...

回答1:

this should do the trick

select ad_id, main, concat(t_0, ', ', t_1, ', ', t_8) as links 
from images 
where main = 0 
group by ad_id having ad_id not in (select ad_id from images where main = 1)
union all
select ad_id, main, concat(t_0, ', ', t_1, ', ', t_8) as links from images where main = 1

回答2:

I think this should work:

SELECT a.id,i.* FROM ads a
INNER JOIN images i 
ON i.ad_id=a.id
WHERE i.main=1        
GROUP BY a.id
LIMIT 10

Edit: I think you could also drop the GROUP BY clause then as there will be a 1 to 1 mapping between A and I

回答3:

You don't need a group by at all here:

SELECT ads.id,images.*
    FROM ads
        INNER JOIN images ON(Images.ad_id=ads.id)
    WHERE Images.Flag=x
    ORDER BY ads.id
    LIMIT 10

If you want to include images without the flag when there is no flag, or return only 1 image per ad even if multiple images have the flag, then the query will be more difficult. You'll have to clarify to get a more targeted answer.

Edit: Something like this might work

SELECT mainImages.Ad_ID2,
        IFNULL(mainImages.T_0,images.T_0) as T0,
        IFNULL(mainImages.T_1,images.T_1) as T1
    FROM images
        inner join 
            (SELECT ads.id as Ad_ID2,images.*
                FROM ads
                    LEFT OUTER JOIN images ON(Images.ad_id=ads.id)
                WHERE Images.Main=1) as mainImages
            on images.Ad_ID=mainImages.Ad_ID2
    ORDER BY mainImages.Ad_ID2
    LIMIT 10

It uses the subquery join to create a list of IDs with image data (only where the main flag is set) then re-joins the images table again and fills in the null image data from the new join if it doesn't already exist in the subquery. As I said, I can't test it here, but I think the general concept is sound.

Edit: Fixed query to use updated tables in question.

Edit: Changed column name as per comment

来源：https://stackoverflow.com/questions/3513262/mysql-problem-with-statement

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