MySQL语句问题 [英] MySQL problem with statement

问题描述

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

不知道如何在图像表中选择带有"main"标志的图像.

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

对于一个aid，最多可以在图像表中包含3张照片，这3张照片之一可以具有main = 1字段.我需要在将main标记设置为1的情况下按优先顺序选择照片.

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');

类似的东西，但是每个ads.id只有1行

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  

需要的输出:

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

推荐答案

这应该可以解决问题

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

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