SQL To Find Most RECENT row Using GROUP BY - sql

Question

SQL newbie hear tearing my hair out trying to work this one out! I have a problem that is similar to this.
I have the following data and all fields are defined as CHARACTER - No DATE or TIME unfortunately thanks to poor design by the original DBA
Surname Name LoginDate LoginTime
Smith John 2014-06-25 13.00
Smith John 2014-06-24 14.00
Smith Susan 2014-06-26 09.00
Smith Susan 2014-06-26 11.30
Jones Bill 2014-06-25 09.30
Jones Bill 2014-06-25 12.30
Jones Bill 2014-06-26 07.00
What i'm trying to get on my output is the most recent log in for each person so I would expect to see
Smith John 2014-06-25 13.00
Smith Susan 2014-06-26 11.30
Jones Bill 2014-06-26 07.00
I've tried different combinations of temporary tables, using CONCAT on the Date and Time and the MAX function but I just keep drawing a blank. I think I know the tools and commands I need to use I just can't seem to string them all together properly.
I know I have to group them by name/surname then somehow combine the date and time in a way that lets me use the MAX function but when I output them I can never seem to get the LoginDate and LoginTime to appear as seperate fields on the output because they're not included in any GROUP BY that I use.
Is anyone able to show me how to do this as I haven't got a lot of hair to start with :)

Answer 1

Try this Query -
With MaxTimeStamp as
(
SELECT Surname, Name, Max(TIMESTAMP(LoginDate, LoginTime)) as LoginDateTime
FROM YourTable
group by Surname, Name
)
select c.Surname, c.Name, d.LoginDate, d.LoginTime,
from MaxTimeStamp c
Join YourTable d
on c.Surname = d.Surname
and c.Name = d.Name
and Date(c.LoginDateTime) = d.LoginDate
and Time(c.LoginDateTime) = d.LoginTime

Answer 2

You don't do this using group by. Here is a method using not exists:
select t.*
from table t
where not exists (select 1
from table t2
where t2.name = t.name and t2.surname = t.surname and
(t2.logindate > t.logindate or
t2.logindate = t.logindate and
t2.logintime > t.logintime
);
This transforms the query to: "Get me all rows from the table where there is no row with the same name and a later login." It would be simpler if the login information were stored as a single datetime.
Also, the above will work in just about all SQL databases. Many databases support window functions which can also be used for this problem.

Answer 3

In addition to Gordon Linoff's answer, if your SQL database supports window functions, you can also use this to get the desired result:
;With Cte As
(
Select *,
Row_Number() Over (Partition By Surname, Name Order By LoginDate Desc, LoginTime Desc) RN
From Table
)
Select Surname, Name, LoginDate, LoginTime
From Cte
Where RN = 1

Answer 4

Why not a group by? i´m missing anything?
Select name, surname, LoginDate, max(LoginTime) from table where (name,surname,login_date) in (select name, surname, max(LoginDate) from table group by name,surname) group by name, surname, LoginDate
If the LoginDate is not a date or similar datatype, it should run i guess...
Br.

Answer 5

I have split it into two parts, to find the newest login per user and do the group by. I then join to the data again to find the record.
select
dt.Surname
, dt.Name
, dt.LoginDate
, dt.LoginTime
from
dataTable dt
join
(select Surname, Name, MAX(LoginDate+LoginTime) ts from #temp group by surname, name) sub
on sub.Name = dt.Name
and sub.Surname = dt.Surname
and dt.LoginDate+dt.LoginTime = ts
WHERE
not sub.Name is null
You can also do it without the need to join, by splitting the timestamp out again.
select
sub.Surname
, sub.Name
, LEFT(ts,10) LoginDate
, RIGHT(ts,5) LoginTime
from
(select Surname, Name, MAX(LoginDate+LoginTime) ts from #temp group by surname, name) sub