Jak połączyć tablice z MySQL z 3 tabel relacji wiele-do-wielu?

Metodą "eazy" jest użycie CONCAT do wygenerowania JSON.
I użyj GROUP_CONCAT, aby połączyć rekordy wielu zespołów w tablicę JSON.
Ta metoda działa również w starszych wersjach MySQL, które nie obsługują tworzenia funkcji JSON.

Zapytanie

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

Wynik

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

demonstracja

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

Edytowano więcej znajomych

Zapytanie

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

Wynik

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

demonstracja

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