How to get Max column with group by using MS SQL? - sql

Question

What I want to query is "get user's last fatal logs". When I query below statement it only returns "username" and "logDate" fields but I also want to get this "logDate"'s corresponding row(I mean logid, logdata);
SELECT user.username, MAX(log.logDate) FROM user
INNER JOIN log ON user.userid = log.userid
WHERE log.logtype = 'fatal'
GROUP BY user.username
My user table;
userid username
-----------------
1 robert
2 ronaldo
log table;
logid logDate logtype userid logdata
----------------------------------------------------------
1 2016-11-28 19:37:53.000 fatal 1 data
2 2016-11-28 22:37:53.000 fatal 1 data
3 2016-11-28 12:37:53.000 fatal 2 data

Answer 1

I will do this using CROSS APPLY(preferred approach with proper index added to Log table)
SELECT *
FROM [USER] u
CROSS apply (SELECT TOP 1 *
FROM log l
WHERE u.userid = l.userid
AND l.logtype = 'fatal'
ORDER BY l.logDate DESC) cs
If the log table is very large then create a Non Clustered Index on Log table to improve the performance
CREATE NONCLUSTERED INDEX NIX_Log_logtype_userid
ON [log] (logtype,userid)
INCLUDE (logid,logDate,logdata)
Another approach using ROW_NUMBER
SELECT *
FROM (SELECT *,
Row_number()OVER(partition BY [USER].username ORDER BY log.logDate DESC) AS rn
FROM [USER]
INNER JOIN log
ON [USER].userid = log.userid
WHERE log.logtype = 'fatal') A
WHERE rn = 1
Another approach using ROW_NUMBER and TOP 1 with ties
SELECT TOP 1 WITH ties *
FROM [USER]
INNER JOIN log
ON [USER].userid = log.userid
WHERE log.logtype = 'fatal'
ORDER BY Row_number()OVER(partition BY [USER].username ORDER BY log.logDate DESC)
Note : All the queries result all the column from both the tables select the required columns

Answer 2

You can use ROW_NUMBER for this:
SELECT user.username,
log.logid, log.logtype, log.logDate, log.logdata
FROM (
SELECT user.username,
log.logid, log.logtype, log.logDate, log.logdata,
ROW_NUMBER() OVER (PARTITION BY user.username
ORDER BY log.logDate DESC) AS rn
FROM user
INNER JOIN log ON user.userid = log.userid
WHERE log.logtype = 'fatal') AS t
WHERE t.rn = 1

Answer 3

A quick option would be to get the max logdate in a subquery. This way, you can select any fields you need from the user table and don't have to aggregate in the outer query. The only issue with this one is that your logdate needs to not have duplicates. If it's a datetime then this isn't likely but you may have duplicates if it's just a date field. Worth checking.
SELECT
u.username
,u.logdate
,u.logid
,u.logdata
FROM user u
INNER JOIN (SELECT
userid
,MAX(logdate) MaxLog
FROM log
WHERE logtype = 'fatal'
GROUP BY userid) l
ON u.userid = l.userid
AND u.logdate = l.MaxLog

Answer 4

WITH MaxLogDate AS (
SELECT user.userid, MAX(log.logDate) logDate FROM user
INNER JOIN log ON user.userid = log.userid
WHERE log.logtype = 'fatal'
GROUP BY user.userid
)
SELECT log.logid, log.logDate, log.logtype, u.userid, u.username
FROM user u
JOIN MaxLogDate m ON u.userid = m.userid
JOIN log ON log.logDate = m.logDate AND log.userid = m.userid
WHERE log.logtype = 'fatal' --This line is optional, may increase the performance.