Find max over multiple columns - sql

Question

I am trying to query a list of meetings from the most recent semester, where semester is determined by two fields (year, semester). Here's a basic outline of the schema:
Otherfields Year Semester
meeting1 2014 1
meeting2 2014 1
meeting3 2013 2
... etc ...
As the max should be considered for the Year first, and then the Semester, my results should look like this:
Otherfields Year Semester
meeting1 2014 1
meeting2 2014 1
Unfortunately simply using the MAX() function on each column separately will try to find Year=2014, Semester=2, which is incorrect. I tried a couple approaches using nested subqueries and inner joins but couldn't quite get something to work. What is the most straightforward approach to solving this?

Answer 1

Using a window function:
SELECT Year, Semester, RANK() OVER(ORDER BY Year DESC, Semester DESC) R
FROM your_table;
R will be a column containing the "rank" of the couple (Year, Semester). You can then use this column as a filter, for instance :
WITH TT AS (
SELECT Year, Semester, RANK() OVER(ORDER BY Year DESC, Semester DESC) R
FROM your_table
)
SELECT ...
FROM TT
WHERE R = 1;
If you don't want gaps between ranks, you can use dense_rank instead of rank.
This answer assumes you use a RDBMS who is advanced enough to offer window functions (i.e. not MySQL)

Answer 2

I wouldn't be surprised if there's a more effecient way to do this (and avoid the duplicate subquery), but this will get you the answer you want:
SELECT * FROM table WHERE Year =
(SELECT MAX(Year) FROM table)
AND Semester =
(SELECT MAX(Semester) FROM table WHERE Year =
(SELECT MAX(Year) FROM table))

Answer 3

Here's Postgres:
with table2 as /*virtual temporary table*/
(
select *, year::text || semester as yearsemester
from table
)
select Otherfields, year, semester
from table2
where (Otherfields, yearsemester) in
(
select Otherfields, max(yearsemester)
from table2
group by Otherfields
)

Answer 4

I've been overthinking this, there's a much simpler way to get this:
SELECT Meeting.year, Meeting.semester, Meeting.otherFields
FROM Meeting
JOIN (SELECT year, semester
FROM Meeting
WHERE ROWNUM = 1
ORDER BY year DESC, semester DESC) MostRecent
ON MostRecent.year = Meeting.year
AND MostRecent.semester = Meeting.semester
(and working Fiddle)
Note that variations of this should work for pretty much all dbs (anything that supports a limiting clause in a subquery); here's the MySQL version, for example:
SELECT Meeting.year, Meeting.semester, Meeting.otherFields
FROM Meeting
JOIN (SELECT year, semester
FROM Meeting
ORDER BY year DESC, semester DESC
LIMIT 1) MostRecent
ON MostRecent.year = Meeting.year
AND MostRecent.semester = Meeting.semester
(...and working fiddle)
Given some of the data in this answer this should be performant for Oracle, and I suspect other dbs as well (given the shortcuts the optimizer is allowed to take). This should be able to replace the use of things like ROW_NUMBER() in most instances where no partitioning clause is provided (no window).

Answer 5

why don't you simply use ORDER BY???
that way, it would be easier to handle and less messy!! :)
SELECT * FROM table
Where Year = (Select Max(Year) from table) /* optional clause to select only 2014*/
Order by Semester ASC, Year DESC, Otherfields; /*numericaly lowest sem first. in case of sem clash, sort by descending year first */
EDIT
In case, you need limited results from 2014, use Limit clause ( for mysql )
SELECT * FROM table
Where Year = (Select Max(Year) from table)
Order by Semester ASC, Year DESC, Otherfields
LIMIT 10;
It will order first, then get the Limit - 10, so u get your limited result set!
This will fetch output like :
Otherfields Year Semester
meeting1 2014 1
meeting2 2014 1
meeting1 2013 1
meeting2 2013 2

Answer 6

Answering my own question here:
This query was run in a stored procedure, so I went ahead and found the maximum year/semester in separate queries before the rest of the query. This is most likely inefficient and inelegant, but it is also the most understandable method- I don't need to worry about other members of my team getting confused by it. I'll leave this question here since it's generally applicable to many other situations, and there appear to be some good answers providing alternative approaches.
-- Find the most recent year.
SELECT MAX(year) INTO max_year FROM meeting;
-- Find the most recent semester in the year.
SELECT MAX(semester) INTO max_semester FROM meeting WHERE year = max_year;
-- Open a ref cursor for meetings in most recent year/semester.
OPEN meeting_list FOR
SELECT otherfields, year, semester
FROM meeting
WHERE year = max_year
AND semester = max_semester;