更高效的MySQL子查询？

Question

我在使用子查询查询查询时遇到问题。如果您愿意，请继续：

示例数据：

CREATE TABLE users (
    user_id INT(10) NOT NULL AUTO_INCREMENT,
    username VARCHAR(64),
    PRIMARY KEY (user_id)
);

CREATE TABLE cars {
    car_id INT(10) NOT NULL AUTO_INCREMENT,
    user_id INT(10),
    caryear YEAR(4),
    carmaker VARCHAR(32),
    carmodel VARCHAR(32),
    PRIMARY KEY (car_id, user_id)
    FOREIGN KEY(`user_id`) references users(`user_id`)
) ENGINE=InnoDB;    

INSERT INTO users (user_id, name) VALUES (1,'Bob'),(2,'John'),(3,'Sally');
INSERT INTO cars (user_id, caryear, carmaker, carmodel) VALUES
(1,'2004','Audi','A4'),
(1,'2006','Toyota','Camry'),
(1,'2014','Jeep','CJ'),
(2,'1998','Acura','CL'),
(2,'2014','Honda','Accord'),
(3,'2011','Jeep','Rubicon')

好的，所以如果我想获得一个用户和他们的汽车列表，我可以做这样的事情：

SELECT
  user_id,
  username,
  (SELECT GROUP_CONCAT(CONCAT(CarYear,' ',CarMaker,' ',CarModel))
   FROM cars
   WHERE cars.user_id = users.user_id
  ) AS Cars
FROM users;

但是，如何获得拥有2004-2014吉普车的用户列表呢？

我已经想出了这个，但我确信有更优雅的方法：

SELECT
  user_id,
  username,
  (SELECT GROUP_CONCAT(CONCAT(CarYear,' ',CarMaker,' ',CarModel))
   FROM cars
   WHERE cars.user_id = users.user_id
   AND CarMaker = 'Jeep'
   AND CarYear BETWEEN 2004 AND 2014
  ) AS Cars
FROM users
HAVING Cars is not null;

这个解决方案的问题在于它没有显示拥有2004-2014吉普车的人拥有的其他车辆。

有什么建议吗？

Answer 1

为任何拥有Jeep 2004-2014的用户显示用户及其所有车辆列表：

SELECT
  u.user_id,
  u.username,
  GROUP_CONCAT(CONCAT(c.CarYear,' ',c.CarMaker,' ',c.CarModel)) AS Cars
FROM users u
INNER JOIN cars c USING (user_id)
INNER JOIN (
  SELECT DISTINCT user_id FROM cars
  WHERE c.CarMaker = 'Jeep' 
    AND c.CarYear BETWEEN 2004 AND 2014) AS j USING (user_id)
GROUP BY u.user_id, u.username;

Answer 2

因为这是一个多对多的关系，所以实际上你需要将cars分成两个表。第一个表格与cars现在相同，只是从中移除user_id。第二个表格（称为user_car）有两列：user_id和car_id。

一旦你这样做，查询很简单：

SELECT user_id FROM user_car WHERE car_id IN
    (SELECT car_id FROM cars
   WHERE CarMaker = 'Jeep'
   AND CarYear BETWEEN 2004 AND 2014
  )

如果您需要直接从users提取数据，请将查询展开为：

SELECT user_id, username FROM users WHERE user_id IN
  (SELECT user_id FROM user_car WHERE car_id IN
    (SELECT car_id FROM cars
   WHERE CarMaker = 'Jeep'
   AND CarYear BETWEEN 2004 AND 2014
  ))

Answer 3

INNER JOIN可以解决问题。

SELECT u.* FROM users u
INNER JOIN cars c ON c.user_id = u.user_id
WHERE CarMaker = 'Jeep' AND CarYear BETWEEN 2004 AND 2014