MySQL多表联接

Question

我有3个表，我正在尝试连接，并获得不同的结果。

CREATE TABLE  `car` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB

mysql> select * from car;

+----+-------+
| id | name  |
+----+-------+
|  1 | acura | 
+----+-------+




CREATE TABLE  `tires` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `tire_desc` varchar(255) DEFAULT NULL,
  `car_id` int(10) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  KEY `new_fk_constraint` (`car_id`),
  CONSTRAINT `new_fk_constraint` FOREIGN KEY (`car_id`) REFERENCES `car` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB

mysql> select * from tires;

+----+-------------+--------+
| id | tire_desc   | car_id |
+----+-------------+--------+
|  1 | front_right |      1 | 
|  2 | front_left  |      1 | 
+----+-------------+--------+


CREATE TABLE  `lights` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `lights_desc` varchar(255) NOT NULL,
  `car_id` int(10) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  KEY `new1_fk_constraint` (`car_id`),
  CONSTRAINT `new1_fk_constraint` FOREIGN KEY (`car_id`) REFERENCES `car` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB

mysql> select * from lights;

+----+-------------+--------+
| id | lights_desc | car_id |
+----+-------------+--------+
|  1 | right_light |      1 | 
|  2 | left_light  |      1 | 
+----+-------------+--------+

这是我的问题。

mysql> SELECT name, group_concat(tire_desc), group_concat(lights_desc)
FROM car 
left join tires on car.id = tires.car_id 
left join lights on car.id = car_id 
group by car.id;

+-------+-----------------------------------------------+-----------------------------------------------+
| name  | group_concat(tire_desc)                       | group_concat(lights_desc)                     |
+-------+-----------------------------------------------+-----------------------------------------------+
| acura | front_right,front_right,front_left,front_left | right_light,left_light,right_light,left_light | 
+-------+-----------------------------------------------+-----------------------------------------------+

我得到了重复的条目，这就是我想要得到的。

+-------+-----------------------------------------------+--------------------------------+
| name  | group_concat(tire_desc) | group_concat(lights_desc) |
+-------+-----------------------------------------------+--------------------------------+
| acura | front_right,front_left | right_light,left_light | 
+-------+-----------------------------------------------+--------------------------------+

我不能在group_concat中使用distinct，因为我可能有想要保留的合法副本。有没有办法使用连接而不是像下面的语句那样使用inner来执行此查询？

SELECT name, 
(select group_concat(tire_desc) from tires where car.id = tires.car_id), 
(select group_concat(lights_desc) from lights where car.id = lights.car_id)
FROM car

另外，如果我使用内部selects，在连接的时候会有性能问题吗？

Answer 1

有没有办法使用joins而不是像下面的语句那样使用inner来执行此查询？

有，但效率不高，可以省略一些预期的重复项：

SELECT name, group_concat(DISTINCT tire_desc), group_concat(DISTINCT lights_desc)
FROM car 
left join tires on car.id = tires.car_id 
left join lights on car.id = car_id 
group by car.id;

另外，如果我使用内部selects，在连接的时候会有任何性能问题吗？

在你的例子中，是的:子选择会更快。

在MyISAM表上，在某些情况下，子选择往往比GROUP BY查询更快，这是因为MyISAM中的GROUP BY由于排序/物化开销而相当昂贵：

Answer 2

也许你必须在第二个中指定你指的是哪个car_id。

SELECT name, group_concat(tire_desc), group_concat(lights_desc)
FROM car 
left join tires on car.id = tires.car_id 
left join lights on car.id = **lights**.car_id 
group by car.id;

我相信在InnoDB表上连接比子查询更快。