JOIN and GROUP_CONCAT with three tables

问题

I have three tables:

users:        sports:           user_sports:

id | name     id | name         id_user | id_sport | pref
---+--------  ---+------------  --------+----------+------
 1 | Peter     1 | Tennis             1 |        1 |    0
 2 | Alice     2 | Football           1 |        2 |    1
 3 | Bob       3 | Basketball         2 |        3 |    0
                                      3 |        1 |    2
                                      3 |        3 |    1
                                      3 |        2 |    0

The table user_sports links users and sports with an order of preference (pref).

I need to make a query that returns this:

id | name  | sport_ids | sport_names
---+-------+-----------+----------------------------
 1 | Peter | 1,2       | Tennis,Football
 2 | Alice | 3         | Basketball
 3 | Bob   | 2,3,1     | Football,Basketball,Tennis

I have tried with JOIN and GROUP_CONCAT but I get weird results.
Do I need to do a nested query?
Any ideas?

回答1:

Its not particularly difficult.

Join the three tables using the JOIN clause.
Use Group_concat on the fields you're interested in.
Don't forget the GROUP BY clause on the fields you're not concatenating or weird things will happen

SELECT u.id, 
       u.Name, 
       Group_concat(us.id_sport order by pref) sport_ids, 
       Group_concat(s.name order by pref)      sport_names 
FROM   users u 
       LEFT JOIN User_Sports us 
               ON u.id = us.id_user 
       LEFT  JOIN sports s 
               ON US.id_sport = s.id 
GROUP  BY u.id, 
          u.Name

DEMO

Update LEFT JOIN for when the user doesn't have entries in User_Sports as per comments

回答2:

I think this is just a simple join and aggregation:

select u.id, u.name, group_concat(s.name order by pref separator ',')
from user_sports us join
     users u
     on us.id_user = u.id join
     sports s
     on us.id_sport = s.id
group by u.id, u.name

来源：https://stackoverflow.com/questions/10956188/join-and-group-concat-with-three-tables

标签

mysql

sql

join

group-concat