如何从多到多关系的3个表中使用MySQL连接数组

Question

我在本地服务器中使用phpMyAdmin创建了一个phpMyAdmin数据库。在这个数据库中，我存储了我的friends.This的名字和最喜欢的NBA球队，这显然是一种多到多的关系。出于这个原因，我在MySQL中运行了下面的脚本来为这个数据库创建适当的表：

CREATE TABLE `friends` (
  `id` int(4) NOT NULL AUTO_INCREMENT,
  `name` varchar(30) NOT NULL,
  PRIMARY KEY (`id`)
)

CREATE TABLE `teams` (
  `id` int(4) NOT NULL AUTO_INCREMENT,
  `name` varchar(30) NOT NULL,
  PRIMARY KEY (`id`)
)

CREATE TABLE `relations` (
  `friends_id` int(4) NOT NULL,
  `teams_id` int(4) NOT NULL,
)

显然，我在这些表中插入了一些值，但为了节省一些空间，我并没有在这里提供大量的源代码。其中一小部分如下：

INSERT INTO `friends` (`id`, `name`)
VALUES
    (1,'David Belton'),
    (2,'Alex James');

INSERT INTO `teams` (`id`, `name`)
VALUES
    (1,'Cleveland Cavaliers'),
    (2,'Boston Celtics');

INSERT INTO `relations` (`friends_id`, `teams_id`)
VALUES
    (1,1),
    (2,1),
    (2,2);

在运行了从数据库获取数据并打印数据的PHP脚本之后，我希望为我的每个朋友提供以下类型的有效的json输出：

{
    "id": "1",
    "name": "Alex James",
    "team": ["Boston Celtics", "Cleveland Cavaliers"] 
}

我怎样才能为MySQL的每个人打造一组最喜欢的团队呢？

我认为在用PHP检索数据之前，最好在MySQL中这样做。

Answer 1

"eazy“方法是使用CONCAT生成JSON。

并使用GROUP_CONCAT将多个团队记录组合到一个JSON数组中。

此方法也适用于不支持创建JSON函数的旧MySQL版本。

查询

SET SESSION group_concat_max_len = @@max_allowed_packet

SELECT 
 CONCAT(
     "{"
   ,     '"id"' , ":" , '"' , friends.id , '"' , ","
   ,     '"name"' , ":" , '"' , friends.name , '"' , ","
   ,     '"team"' , ":" , "["
                              , GROUP_CONCAT('"', teams.name, '"')
                        , "]"
   , "}"
   ) AS json
FROM 
 friends 
INNER JOIN 
 relations 
ON 
 friends.id = relations.friends_id
INNER JOIN
 teams 
ON
 relations.teams_id = teams.id
WHERE 
 friends.id = 1

结果

|                                                            json |
|-----------------------------------------------------------------|
| {"id":"1","name":"David Belton","team":["Cleveland Cavaliers"]} |

演示

http://www.sqlfiddle.com/#!9/4cd244/19

编辑了更多的朋友

查询

SET SESSION group_concat_max_len = @@max_allowed_packet

SELECT
  CONCAT(
      "["
    , GROUP_CONCAT(json_records.json) # combine json records into a string
    , "]"
  )  AS json
FROM (

  SELECT 
     CONCAT(
       "{"
     ,     '"id"' , ":" , '"' , friends.id , '"' , ","
     ,     '"name"' , ":" , '"' , friends.name , '"' , ","
     ,     '"team"' , ":" , "["
                              , GROUP_CONCAT('"', teams.name, '"')
                          , "]"
     , "}"
     ) AS json 
  FROM 
    friends 
  INNER JOIN 
    relations 
  ON 
    friends.id = relations.friends_id
  INNER JOIN
    teams 
  ON
    relations.teams_id = teams.id
  WHERE 
    friends.id IN(SELECT id FROM friends) #select the friends you need or just simply friends.id IN(1, 2)
  GROUP BY
     friends.id
) 
 AS json_records

结果

|                                                                                                                                             json |
|--------------------------------------------------------------------------------------------------------------------------------------------------|
| [{"id":"1","name":"David Belton","team":["Cleveland Cavaliers"]},{"id":"2","name":"Alex James","team":["Boston Celtics","Cleveland Cavaliers"]}] |

演示

http://www.sqlfiddle.com/#!9/4cd244/61