如何通过3个多对多关系表将数组与MySQL连接

Question

我在本地服务器中使用phpMyAdmin创建了一个mySQL数据库。在这个数据库中，我存储了我的朋友的名字和喜欢的NBA球队，这显然是多对多的关系。由于这个原因，我在MySQL中运行以下脚本来为此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，但是为了节省空间，此处没有提供大量源代码。其中的一小部分如下：

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);

运行从数据库中获取数据并打印它们的PHP脚本后，我想为我的每个朋友提供以下有效的json输出：

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

如何使用MySQL为每个人组成这个最喜欢的团队？

PS我假设在使用PHP检索数据之前最好在MySQL中完成此操作。

Answer 1

“简便”方法是使用CONCAT生成JSON。
并使用GROUP_CONCAT将多个团队记录合并到一个JSON数组中。
此方法也可在不支持创建JSON函数的旧MySQL版本中使用。

询问

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

编辑了更多朋友

询问

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