SQL Get records with max value for each group - sql

Question

I have 2 tables Journal and Users
Journal looks like this:
TransTime
RegNumber
UserID
5/26/2022 11:00:00
101
3
5/26/2022 11:30:00
102
2
5/26/2022 13:00:00
101
5
5/26/2022 14:30:00
103
4
5/26/2022 15:00:00
102
1
Users table
UserID
Name
1
Ross
2
Rachel
3
Chandler
4
Monica
5
Joey
What I would like to do is get a table of the Registers and their most recent user names. This should seem very simple. But since I am joining tables on the userID, I am getting all 5 records on the first table. But it should look like this:
RegNumber
LastUser
101
Joey
102
Ross
103
Monica
I have tried a variety of solutions but haven't found the right one. Any help is appreciated.

Answer 1

You can use a temptable or cte structure to rank your data based on RegNo and Trantime like below, then retrieve the most updated users for each journal:
CREATE TABLE #Journals (TranTime DATETIME, RegNo INT, UserId INT)
CREATE TABLE #Users (UserId INT, UserName NVARCHAR(100))
INSERT INTO #Users VALUES(1,'Ross'),(2,'Rachel'),(3,'Chandler'),(4,'Monica'),(5,'Joey')
INSERT INTO #Journals VALUES ('5/26/2022 11:00:00',101,3),('5/26/2022 11:30:00',102,2),
('5/26/2022 13:00:00',101,5),('5/26/2022 14:00:00',103,4),('5/26/2022 15:00:00',102,1)
;WITH cte as (
SELECT *,rn=ROW_NUMBER() OVER (PARTITION BY RegNo ORDER BY TranTime DESC)
FROM #Journals
)
SELECT RegNo, u.UserName
FROM cte
INNER JOIN #Users u ON u.UserId = cte.UserId
WHERE rn=1 --since sort by TranTime is descending, it'll give you the latest user for each specific RegNo
ORDER BY RegNo
Tested and it works on SQL Server 2016.

Answer 2

if you start with an inner join of the max transtime per regNumber, then join with user table:
Select J.RegNumber, U.Name
From Journal J
Inner join
(Select Max(TransTime) as TransTime, RegNumber
From Journal
Group by RegNumber) J2 on J.TransTime = J2.TransTime and J.RegNumber = J2.RegNumber
Inner join
Users U on J.UserID = U.UserID

Answer 3

Here is an option using a CTE:
;with cte as
(
Select RegNumber,
UserID = max(UserID)
From journal
group by RegNumber
)
Select RegNumber = C.RegNumber,
LastUser = U.Name
From cte C
Join users U ON U.Userid = C.UserID
order by C.RegNumber

Answer 4

This answer is not, at its core, substantively different from the others. However, in terms of being helpful to the target audience it's more readable, more self-documenting, and more standard in terms of formatting.
Sidebar: This SQL takes the data design at face value, as a given, with the implicit assumption that TransTime is the PK or at least uniquely indexed, possibly in conjunction with RegNumber. Bottom line, it would be good to have a little more info about the key structure along with the original question.
WITH LatestEntries AS
(
SELECT
MAX(TransTime) AS LatestTimeForReg
,RegNumber
FROM
Journal
GROUP BY
RegNumber
)
SELECT
J.RegNumber
,U.[Name] AS LastUser
FROM
LatestEntries LE
INNER JOIN Journal J ON LE.LatestTimeForReg = J.TransTime AND LE.RegNumber = J.RegNumber
INNER JOIN Users U ON J.UserID = U.UserID
ORDER BY
J.RegNumber
;

Answer 5

select u.Name, j.*
from journal j
inner join (
select max(TransTime) last_update, RegNumber
from journal
group by RegNumber
) t1
inner join j.RegNumber = t1.RegNumber
and t1.last_update = j.TransTime
left join Users_Journal uj on j.UserID= uj.UserID