selecting only distinct row based on highest timestamp on a join - mysql

Question

I perform a join on these two tables in mysql
Challenges table
challenge_ID(int) |to_user(int)|from_user(int)|timestamp
Users table
iduser(int)|email(string)
My join query is this :
Select distinct u.email,c.challenge_id,c.status,c.timestamp from
test.challenges c join test.users u
on
c.to_user=u.iduser
where
c.from_user=9 and (c.status='open' || c.status='rejected')
Order by
c.timestamp DESC
The result what I get from this query is
email |challenge_id| status |timestamp (Descending)
Dan21#rab.edu 5 open 2015-12-09 21:20:26
tommy52#gump.com 4 open 2015-12-09 21:10:22
Dan21#rab.edu 1 rejected 2015-12-08 12:27:00
Notice how Dan21#rab.edu is repeated twice, I want it to display only once and the one displayed should have the latest timestamp i.e.
email |challenge_id| status |timestamp (Descending)
Dan21#rab.edu 5 open 2015-12-09 21:20:26
tommy52#gump.com 4 open 2015-12-09 21:10:22

Answer 1

You should write your query as:
Select u.email, c.challenge_id, c.status, c.timestamp
from test.challenges c join
test.users u
on c.to_user = u.iduser
where c.from_user = 9 and c.status in ('open', 'rejected')
Order by c.timestamp DESC;
Don't use select distinct unless necessary. Then you can do what you want in various ways. Because you have a join and other conditions, I think variables might be the simplest way:
select cu.*
from (Select u.email, c.challenge_id, c.status, c.timestamp,
(#rn := if(#e = u.email, #rn + 1,
if(#e := u.email, 1, 1)
)
) as rn
from test.challenges c join
test.users u
on c.to_user = u.iduser cross join
(select #e := '', #rn := 0) params
where c.from_user = 9 and c.status in ('open', 'rejected')
order by u.email, c.timestamp DESC
) cu
where rn = 1;
EDIT:
I thought the above worked with join and the variables in a single query. But, sometimes MySQL gets confused and you need to use a subquery with variables:
select cu.*
from (select cu.*,
(#rn := if(#e = u.email, #rn + 1,
if(#e := u.email, 1, 1)
)
) as rn
from (Select u.email, c.challenge_id, c.status, c.timestamp,
from test.challenges c join
test.users u
on c.to_user = u.iduser
where c.from_user = 9 and c.status in ('open', 'rejected')
order by u.email, c.timestamp DESC
) cu cross join
(select #e := '', #rn := 0) params
) cu
where rn = 1;

Answer 2

I figure out a query for this which works , not sure if its foolproof or most efficient way to do this
Here it is tho
Select distinct u.email,c.challenge_id,c.status,c.timestamp
from
(select ch.challenge_id,ch.status,ch.from_user,ch.to_user,timestamp,
(select Max(timestamp) from challenges
where to_user=ch.to_user
and from_user=9
) as latest
from test.challenges ch
)c
join test.users u
on
c.to_user=u.iduser
and
c.latest=c.timestamp
where (c.status='open' || c.status='rejected')