How to select single row based on the max value in multiple rows [duplicate]

Answer 1

SELECT cam.*
FROM 
      ( SELECT DISTINCT name
        FROM ca 
      ) AS cadistinct
  JOIN 
      ca AS cam
    ON ( cam.name, cam.major, cam.minor, cam.revision )
     = ( SELECT name, major, minor, revision
         FROM ca
         WHERE name = cadistinct.name
         ORDER BY major DESC
                , minor DESC
                , revision DESC
         LIMIT 1
       )

---

This will work in MySQL (current versions) but I woudn't recommend it:

SELECT *
FROM 
    ( SELECT name, major, minor, revision
      FROM ca
      ORDER BY name
             , major DESC
             , minor DESC
             , revision DESC
    ) AS tmp
GROUP BY name

Answer 2

If there are numbers in those columns, you could come up with some kind of a formula that will be unique and well ordered for the major, minor, revision values. E.g. if the numbers are less than 10, you could just append them as strings, and compare them, like:

select name, major, minor, revision, 
       concat(major, minor, revision) as version
from versions

If they are numbers that will not be larger than 100, you could do something like:

select name, major, minor, revision, 
       (major * 10000 + minor * 100 + revision) as version
from versions

You could than just get the max of version grouped by name, like this:

select name, major, minor, revision 
from (
    select name, major, minor, revision, 
           (major * 10000 + minor * 100 + revision) as version
    from versions) v1
where version = (select max (major * 10000 + minor * 100 + revision) 
                 from versions v2 
                 where v1.name = v2.name)

Answer 3

The way I try to solve SQL problems is to take things step by step.

• You want the maximum revision for the maximum minor version corresponding to the maximum major version for each product.

The maximum major number for each product is given by:

SELECT Name, MAX(major) AS Major FROM CA GROUP BY Name;

The maximum minor number corresponding to the maximum major number for each product is therefore given by:

SELECT CA.Name, CA.Major, MAX(CA.Minor) AS Minor
  FROM CA
  JOIN (SELECT Name, MAX(Major) AS Major
          FROM CA
         GROUP BY Name
       ) AS CB
    ON CA.Name = CB.Name AND CA.Major = CB.Major
 GROUP BY CA.Name, CA.Major;

And the maximum revision (for the maximum minor version number corresponding to the maximum major number for each product), therefore, is given by:

SELECT CA.Name, CA.Major, CA.Minor, MAX(CA.Revision) AS Revision
  FROM CA
  JOIN (SELECT CA.Name, CA.Major, MAX(CA.Minor) AS Minor
          FROM CA
          JOIN (SELECT Name, MAX(Major) AS Major
                  FROM CA
                 GROUP BY Name
               ) AS CB
            ON CA.Name = CB.Name AND CA.Major = CB.Major
         GROUP BY CA.Name, CA.Major
       ) AS CC
    ON CA.Name = CC.Name AND CA.Major = CC.Major AND CA.Minor = CC.Minor
 GROUP BY CA.Name, CA.Major, CA.Minor;

Tested - it works and produces the same answer as Andomar's query does.

---

Performance

I created a bigger volume of data (11616 rows of data), and ran a benchmark timing of Andomar's query against mine - target DBMS was IBM Informix Dynamic Server (IDS) version 11.70.FC2 running on MacOS X 10.7.2. I used the first of Andomar's two queries since IDS does not support the comparison notation in the second one. I loaded the data, updated statistics, and ran the queries both with mine followed by Andomar's and with Andomar's followed by mine. I also recorded the basic costs reported by the IDS optimizer. The result data from both queries were the same (so the queries are both accurate - or equally inaccurate).

Table unindexed:

Andomar's query                           Jonathan's query
Time: 22.074129                           Time: 0.085803
Estimated Cost: 2468070                   Estimated Cost: 22673
Estimated # of Rows Returned: 5808        Estimated # of Rows Returned: 132
Temporary Files Required For: Order By    Temporary Files Required For: Group By

Table with unique index on (name, major, minor, revision):

Andomar's query                           Jonathan's query
Time: 0.768309                            Time: 0.060380
Estimated Cost: 31754                     Estimated Cost: 2329
Estimated # of Rows Returned: 5808        Estimated # of Rows Returned: 139
                                          Temporary Files Required For: Group By

As you can seen, the index dramatically improves the performance of Andomar's query, but it still seems to be more expensive on this system than my query. The index gives a 25% time saving for my query. I'd be curious to see comparable figures for the two versions of Andomar's query on comparable volumes of data, with and without the index. (My test data can be supplied if you need it; there were 132 products - the 3 listed in the question and 129 new ones; each new product had (the same) 90 version entries.)

The reason for the discrepancy is that the sub-query in Andomar's query is a correlated sub-query, which is a relatively expensive process (dramatically so when the index is missing).

Answer 4

Update3 variable group_concat_max_len has a minvalue = 4 so we can't use it. But you can:

select 
  name, 
  SUBSTRING_INDEX(group_concat(major order by major desc),',', 1) as major, 
  SUBSTRING_INDEX(group_concat(minor order by major desc, minor desc),',', 1)as minor, 
  SUBSTRING_INDEX(group_concat(revision order by major desc, minor desc, revision desc),',', 1) as revision
from your_table
group by name;

this was tested here and no, the previous version does not provide wrong results, it had only the problem with number of concatenated values.

Answer 5

Am I the only one thinking that the greatest version is the one with the highest revision?

So,

select a.name, a.major, a.minor, a.revision
from table a
where a.revision = (select max(b.revision) from table b where b.name = a.name)

Answer 6

It allows max three digits per part of version number. If you want to use more digits then add two zeros to major multiplication an one zero to minor multiplication for each digit (I hope it's clear).

select  t.* 
from yourTable t
join (
    select name, max(major * 1000000 + minor * 1000  + revision) as ver
    from yourTable 
    group by name
) t1 on t1.ver = (t.major * 1000000 + t.minor * 1000  + t.revision)

Result:

name    major   minor   revision
p1      1       1       4
p2      2       5       0
p3      3       4       4