Mysql Unique records, where multiple records exist - mysql

Question

I am struggling with a Mysql call and was hoping to borrow your expertise.
I believe that what I want may only be possible using two selects and I have not yet done one of these and am struggling to wrap my head around this.
I have a table like so:
+------------------+----------------------+-------------------------------+
| username | acctstarttime | acctstoptime |
+------------------+----------------------+-------------------------------+
| bill | 22.04.2014 | 23.04.2014 |
+------------------+----------------------+-------------------------------+
| steve | 16.09.2014 | |
+------------------+----------------------+-------------------------------+
| fred | 12.08.2014 | |
+------------------+----------------------+-------------------------------+
| bill | 24.04.2014 | |
+------------------+----------------------+-------------------------------+
I wish to select only unique records from the username column ie I only want one record for bill and I need the one with most recent start_date, providing they were weren't in the last three months (end_date is not important to me here) else I do not want any data. In summary I just need anyone where there most recent start date is over 3 months old.
The command I am using currently is:
SELECT DISTINCT(username), ra.acctstarttime AS 'Last IP', ra.acctstoptime
FROM radacct AS ra
WHERE ra.acctstarttime < DATE_SUB(now(), interval 3 month)
GROUP BY ra.username
ORDER BY ra.acctstarttime DESC
However, this simply gives me details about the date_start for that particular customer where they had a start date over 3 months ago.
I have tired a few other combinations of this and have tried a command with a double select but I'm currently hitting brick walls. Any help or a push in the right direction would be much appreciated.
Update
I have created the following:
http://sqlfiddle.com/#!2/f47b2/1
Effectively I should only see 1 row when the query is as it should be. This would be the row for bill. As he is the only one that does not have a start date within the last three months. The result I would expect to see is the following:
24 bill April, 11 2014 12:11:40+0000 (null)
As this is the latest start date for bill, but this start date is not within the last three months. Hopefully this will help clarify. Many thanks for your help thus far.
http://sqlfiddle.com/#!2/f47b2/14
This is another example. If the acctstartdate for bill would show as the April entry, then I could add my where clause for the last three months and this would give me my desired result.

Answer 1

SQLFiddle
http://sqlfiddle.com/#!2/444432/9 (MySQL 5.5)
I am looking at the question in 2 ways based on the current text:
I only want one record for bill and I need the one with most recent start_date, providing they were in the last three months (end_date is not important to me here) else I do not want any data
Structure
create table test
(
username varchar(20),
date_start date
);
Data
Username date_start
--------- -----------
bill 2014-09-25
bill 2014-09-22
bill 2014-05-26
andy 2014-05-26
tim 2014-09-25
tim 2014-05-26
What we want
Username date_start
--------- -----------
bill 2014-09-25
tim 2014-09-25
Query
select *
from test a
inner join
(
select username, max(date_start) as max_date_start
from test
where date_start > date_sub(now(), interval 3 month)
group by username
) b
on
a.username = b.username
and a.date_start = b.max_date_start
where
date_start > date_sub(now(), interval 3 month)
Explanation
For the most recent last 3 months, let's get maximum start date for each user. To limit the records to the latest 3 months we use where date_start > date_sub(now(), interval 3 month) and to find the maximum start date for each user we use group by username.
We, then, join main data with this small subset based on user and max date to get the desired result.
Another angle
If we desire to NOT look at the latest 3 months and instead find the most recent date for each user, we would be looking at this kind of data:
What we want
Username date_start
--------- -----------
bill 2014-05-26
tim 2014-05-26
andy 2014-05-26
Query
select *
from test a
inner join
(
select username, max(date_start) as max_date_start
from test
where date_start < date_sub(now(), interval 3 month)
group by username
) b
on
a.username = b.username
and a.date_start = b.max_date_start
where
date_start < date_sub(now(), interval 3 month)
Hopefully you can change these queries to your liking.
EDIT
Based on your good explanation, here's the query
SQLFiddle: http://sqlfiddle.com/#!2/f47b2/17
select *
from activity a
-- find max dates for users for records with dates after 3 months
inner join
(
select username, max(acctstarttime) as max_date_start
from activity
where acctstarttime < date_sub(now(), interval 3 month)
group by username
) b
on
a.username = b.username
and a.acctstarttime = b.max_date_start
-- find usernames who have data in the recent three months
left join
(
select username, count(*)
from activity
where acctstarttime >= date_sub(now(), interval 3 month)
group by username
) c
on
a.username = c.username
where
acctstarttime < date_sub(now(), interval 3 month)
-- choose users who DONT have data from recent 3 months
and c.username is null
Let me know if you would like me to add explanation

Answer 2

Try this:
select t.*
from radacct t
join (
select ra.username, max(ra.acctstarttime) as acctstarttime
from radacct as ra
WHERE ra.acctstarttime < DATE_SUB(now(), interval 3 month)
) s on t.username = s.username and t.acctstarttime = s.acctstarttime
SQLFiddle