Question

i would like to fetch data from table a,b,c but order by most recent data of table response

table casework has this structure ( simplified):

casework_id | problem   | user_id          
------------+-----------+-------
1           | Problem1  | 1  
2           | Problem2  | 2
3           | Problem3  | 1
4           | Problem4  | 3

table user has this structure ( simplified):

user_id | name         
--------+-----------------
1       | peter  
2       | Sam  
3       | Tom  
4       | Steve

table response has this structure ( simplified):

response_id | response   | casework_id | created           
------------+-----------+--------------+-------
1           | responce1  |  1      | 2012-10-14 11:28:31
2           | responce2  |  1      | 2012-9-10 11:28:31 
3           | responce3  |  1      | 2012-9-2 11:28:31
4           | responce4  |  3      | 2012-8-3 11:28:31
4           | responce5  |  3      | 2012-8-2 11:28:31

I am looking the query to fetch data order by latest responce and group by casework_id

I. e. required out put is 

casework_id | problem   | name  | responce  | created        
------------+-----------+-------+-----------+---------
1           | Problem1  | peter | responce1 | 2012-10-14 11:28:31
2           | Problem2  | Sam   | Null      | Null
3           | Problem3  | peter | responce4 | 2012-8-3 11:28:31 
4           | Problem4  | Tom   | Null      | Null

I would be most grateful if one of you kind people could point me in the right direction.

Answer 1

You can use the following:

select c.casework_id,
  c.problem,
  u.name,
  r2.response,
  r1.created
from casework c
left join user u
  on c.user_id = u.user_id
left join
(
  select max(created) created, casework_id
  from response r
  group by casework_id
) r1
  on c.casework_id = r1.casework_id
left join response r2
  on r1.created = r2.created
  and r1.casework_id = r2.casework_id

See SQL Fiddle with Demo

If you want to include both the user that created the casework and then who responsed, then you will want to join on the user table twice:

select c.casework_id,
  c.problem,
  u1.name CreatedByName,
  r2.response,
  r1.created,
  u2.name ReponseName
from casework c
left join user u1
  on c.user_id = u1.user_id
left join
(
  select max(created) created, casework_id
  from response r
  group by casework_id
) r1
  on c.casework_id = r1.casework_id
left join response r2
  on r1.created = r2.created
  and r1.casework_id = r2.casework_id
left join user u2
  on r2.user_id = u2.user_id

See SQL Fiddle with demo