相关子查询总是引发错误，找不到原因

Question

这里是可能的应用数据库tables

CREATE TABLE IF NOT EXISTS `Cars` (
  `car_id` int(11) NOT NULL AUTO_INCREMENT,
  `type` int(11) NOT NULL,
  `price` int(11) NOT NULL,
  `insertionDate` datetime NOT NULL,
  PRIMARY KEY (`car_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1;

 CREATE TABLE IF NOT EXISTS `newCars` (
  `car_id` int(11) NOT NULL, //Only new cars have a dealer (Cars.type = 1)
  `dealer_id` int(11) NOT NULL,
  PRIMARY KEY (`car_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE IF NOT EXISTS `CarsPhotos` (
  `photo_id` int(11) NOT NULL AUTO_INCREMENT,
  `car_id` int(11) NOT NULL,
  `name` varchar(255) NOT NULL,
  `insertionDate` datetime NOT NULL,
  PRIMARY KEY (`photo_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1;

CREATE TABLE IF NOT EXISTS `Dealers` (
  `dealer_id` int(11) NOT NULL AUTO_INCREMENT,
  `dealerName` varchar(255) NOT NULL,
  `website` varchar(255) NOT NULL,
  `insertionDate` datetime NOT NULL,
  PRIMARY KEY (`dealer_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1;

CREATE TABLE IF NOT EXISTS `dealersPhotos` (
  `photo_id` int(11) NOT NULL AUTO_INCREMENT,
  `dealer_id` int(11) NOT NULL,
  `name` varchar(255) NOT NULL,
  `insertionDate` datetime NOT NULL,
  PRIMARY KEY (`photo_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1;

我正在尝试查找至少有一张相关照片（汽车照片或经销商照片）的汽车，如下所示：

搜索操作必须在一次查询中完成
检查汽车是否至少有一张照片 carPhotosCount。
if carPhotosCount = 0 比检查汽车是否是新车 (Cars.type = 1)
如果汽车是新车，那么从 newCars table
检查经销商是否至少有一张图片 dealerPhotosCount。

为此，我用两种方式编写了 SQL 查询 (MySQL)，它们都引发了相关错误。

SELECT car_id ,
    FROM Cars c
    WHERE
      CASE
        WHEN
          (SELECT count(*) as carPhotosCount FROM CarsPhotos WHERE CarsPhotos.car_id = c.car_id HAVING carPhotosCount > 0) THEN 1
        ELSE
          CASE
            WHEN type = 1 THEN
              CASE
                WHEN
                  (SELECT count(*) as dealerPhotosCount FROM dealersPhotos
                  JOIN  newCars ON  newCars.car_id = c.car_id
                  WHERE dealersPhotos.dealer_id = newCars.dealer_id HAVING dealerPhotosCount > 0) THEN 1
                ELSE 0
              END
            ELSE 0
          END
      END

查询 2：

SELECT car_id ,
      CASE
        WHEN type = 1 THEN (SELECT newCars.dealer_id FROM newCars WHERE newCars.car_id = c.car_id)
        ELSE null
      END deal_id
    FROM Cars c
    WHERE
      CASE
        WHEN (SELECT count(*) as dealerPhotosCount FROM dealersPhotos WHERE dealersPhotos.dealer_id = deal_id HAVING dealerPhotosCount > 0) THEN 1
        ELSE
          CASE
            WHEN (SELECT count(*) as carPhotosCount FROM CarsPhotos WHERE CarsPhotos.car_id = c.car_id HAVING carPhotosCount > 0) THEN 1
            ELSE 0
          END
      END

Answer 1

我想你只想要 exists 和 limit:

SELECT car_id ,
       (CASE WHEN type = 1
            THEN (SELECT nc.dealer_id FROM newCars nc WHERE nc.car_id = c.car_id LIMIT 1)
        END) as deal_id
FROM Cars c
WHERE EXISTS (SELECT 1
              FROM dealersPhotos dp
              WHERE dp.dealer_id = c.deal_id
             ) OR
      EXISTS (SELECT 1 
              FROM CarsPhotos cp
              WHERE cp.car_id = c.car_id
             ) ;

查询中没有明显的重大错误。但是，SELECT 中的子查询可以 return 多行，这通常会导致错误。

编辑：

您似乎想在外部查询中加入经销商，然后将逻辑合并到 WHERE 子句中。一种方法是在 WHERE 子句中重复表达式：

SELECT c.car_id,
       (CASE WHEN c.type = 1 THEN nc.dealer_id END) as deal_id
FROM Cars c LEFT JOIN
     newCars nc
     ON nc.car_id = c.car_id 
WHERE EXISTS (SELECT 1
              FROM dealersPhotos dp
              WHERE dp.dealer_id = (CASE WHEN c.type = 1 THEN nc.dealer_id END)
             ) OR
      EXISTS (SELECT 1 
              FROM CarsPhotos cp
              WHERE cp.car_id = c.car_id
             ) ;

相关子查询总是引发错误，找不到原因

Correlated subqueries always raise error, Can't find why

mysql

sql

database

correlation