MySQL View Problems

Question

I have the following tables, users which is self explanatory and answers which contains a list of responses on a given date for a specific user.

users
-----
ID   FIRST_NAME   LAST_NAME
1    Joe          Bloggs  
2    Fred         Sexy
3    Jo           Fine
4    Yo           Dude
5    Hi           There

answers
-------
ID   CREATED_AT   RESPONSE   USER_ID
1    2011-01-01   3          1
2    2011-01-01   4          2
3    2011-01-02   5          5

My aim is to build a view which would output the following:

USER_ID   CREATED_AT   RESPONSE
1         2011-01-01   3
2         2011-01-01   4
3         2011-01-01   NULL
4         2011-01-01   NULL
5         2011-01-01   NULL
1         2011-01-02   NULL
2         2011-01-02   NULL
3         2011-01-02   NULL
4         2011-01-02   NULL
5         2011-01-02   5

I have been trying to do this in one SELECT statement but I don't believe it is possible, maybe I'm missing something? I can accomplish the output with multiple statements but I'm looking for a more elegant method which can sit in a view (or multiple views).

Thanks in advance!

Answer 1

This should work, but I recommend against using it unless the answers table will always be fairly small:

select u.id user_id,
       a.created_at,
       max(case when a.user_id = u.id then response end) response
from users u
cross join answers a
group by u.id, a.created_at

Answer 2

select users.id as user_id, created_at, response from users
  left outer join answers on users.id = answers.user_id
  order by created_at, users.id

Answer 3

try this

select t3.user_id, t3.created_at, a.response
from 
(select t2.user_id as user_id, t1.created_at as created_at, null
from
(select distinct created_at
from answers) t1, users t2) t3 answers a
where t3.user_id = a.user_id and t3.created_at = a.created_at

for nulls, I guess left outer join will work

select t3.user_id, t3.created_at, a.response
from 
(select t2.user_id as user_id, t1.created_at as created_at, null
from
(select distinct created_at
from answers) t1, users t2) t3 LEFT OUTER JOIN answers a
ON t3.user_id = a.user_id and t3.created_at = a.created_at

Answer 4

This does the trick but instead of returning NULL for missing responses, it returns 0.

  select 
    distinct u.id, a.created_at, MAX(IF(u.id=a.user_id, a.response, 0)) response
  from users u, answers a
    group by id, created_at
    order by created_at, u.id