HAVING - GROUP BY to get the latest record

问题

I have a table called RESULTS like this :

RESULTS_IDN    NAME    SUBJECT    YEAR    QUALIFIED
1              MARK    ENGLISH    1989    N
3              MARK    ENGLISH    1991    N
5              MARK    ENGLISH    1993    Y
7              MARK    ENGLISH    1995    N
2              MARK    MATH       1990    N
5              MARK    MATH       1993    N
6              MARK    MATH       1995    Y
4              MARK    SCIENCE    1991    N
9              MARK    SCIENCE    1997    Y

I need to know the Qualification Status of the CANDIDATE for a SUBJECT for the LATEST exam he has written , how do I write a query for this (ORACLE/MSSQL) ?

For example Input

NAME,SUBJECT  OUTPUT NAME IDN SUBJECT YEAR Q
MARK,ENGLISH  OUTPUT MARK 7   ENGLISH 1995 N 
MARK SCIENCE  OUTPUT MARK 9   SCIENCE 1997 Y
MARK MATH     OUTPUT MARK 6   MATH    1995 Y

I know of one way to solve this .

(SELECT NAME SUBJECT YEAR MAX(YEAR) YEAR
FROM RESULTS WHERE NAME = 'MARK' AND SUBJECT ='MATH'
GROUP BY NAME SUBJECT YEAR) LATEST 

Join the above table back on IDN to the same table and I can get the results . But this is double work . Is there anyway I can club the MAX(YEAR) and get the CORRESPONDING YEAR using HAVING CLAUSE or something ? I need 2 operations on the GROUP BY data , one Latest , and the corresponidng Qualified status .

PS : Ofcourse there are records for 100 candidates like this in the DB .

Update : This question is also categorized as greatest-n-per-group problem as per answer 2. Interesting to know it is a classified problem in DB .

回答1:

In both Oracle and SQL Server you can use the analytic/windowing functions RANK() or ROW_NUMBER() to achieve this:

select *
  from ( select a.*
              , rank() over ( partition by name, subject order by year desc ) rnk
           from ... a
                )
 where rnk = 1

RANK() will return 1 for every row that is the newest per name and subject, ROW_NUMBER() will return a random row.

In Oracle alone you can use KEEP to give you the same result:

select name, subject, max(year) as year
     , max(qualified) keep (dense_rank first order by year desc) as qualified
  from ...
 group by name, subject

回答2:

This is all-the-time reinvented problem of greatest-n-per-group:

SELECT t1.*
FROM RESULTS AS t1
LEFT JOIN RESULTS AS t2
  ON t1.NAME = t2.NAME AND t1.SUBJECT = t2.SUBJECT AND t1.YEAR < t2.YEAR
WHERE t2.NAME IS NULL

来源：https://stackoverflow.com/questions/17380456/having-group-by-to-get-the-latest-record