MySQL使用Group By来限制结果

Question

Okay, so I am trying to perform a query that has 4 tables, users, events, event_roles, user_event_role.

The users can fill multiple roles. What i am trying to do is get a result that looks more like this: User, event, Role(s)

So if user 'Bill' is associated with event 'Meeting' and 'Bill' Fills multiple roles instead of getting a result like this:

user   event     role
--------------------------
bill   Meeting   admin
bill   Meeting   director

how would I get my result to be like this

user   event     role    role
----------------------------------
bill   Meeting   admin   director

Here is a query that I'm trying to build off of.

Select * 
  FROM `users` u 
LEFT JOIN `event_role` er ON u.user_id = er.user_id 
LEFT JOIN `events` e ON er.event_id = e.event_id

Answer 1

The result you seek is not possible.

However there is something close:

SELECT
  user,
  event,
  group_concat(role SEPARATOR ',') as roles
FROM
  `users` u 
  LEFT JOIN `event_role` er
    ON u.user_id = er.user_id 
  LEFT JOIN `events` e
    ON er.event_id = e.event_id
GROUP BY u.user_id

which would yield:

user   event     roles
----------------------
bill   Meeting   admin,director

In either case you would need to adjust your logic to parse it correctly.

Answer 2

You cannot get such result, because you don't know how many roles there might be (ie columns count), but you can use GROUP_CONCAT that way:

SELECT *,
GROUP_CONCAT(event_roles.role SEPARATOR ',') as roles
FROM users
LEFT JOIN event_role USING(user_id)
LEFT JOIN events USING(user_id)
GROUP BY user_id

Using this query you will get all roles concatonated with , . But be aware of limitation of GROUP_CONCAT , the default value is set to 1024 characters which might not be enough (see my.cnf ).

Answer 3

Use Group_concat ( http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html ) like this. To show that quickly, I'm using a database from the excellent MySQL intro book by Ben Forta ( http://www.forta.com/books/0672327120/ ), no affiliation.

SELECT v.vend_id, GROUP_CONCAT(p.prod_id)
FROM products p 
JOIN  vendors v ON v.vend_id= p.vend_id
JOIN  orderitems oi ON oi.prod_id = p.prod_id
GROUP BY v.vend_id;