select rows satisfying some criteria and with maximum value in a certain column

问题

I have a table of metadata for updates to a software package. The table has columns id, name, version. I want to select all rows where the name is one of some given list of names and the version is maximum of all the rows with that name.

For example, given these records:

+----+------+---------+
| id | name | version |
+----+------+---------+
| 1  | foo  | 1       |
| 2  | foo  | 2       |
| 3  | bar  | 4       |
| 4  | bar  | 5       |
+----+------+---------+

And a task "give me the highest versions of records "foo" and "bar", I want the result to be:

+----+------+---------+
| id | name | version |
+----+------+---------+
| 2  | foo  | 2       |
| 4  | bar  | 5       |
+----+------+---------+

What I come up with so far, is using nested queries:

SELECT * 
  FROM updates 
  WHERE (
    id IN (SELECT id 
             FROM updates 
             WHERE name = 'foo' 
             ORDER BY version DESC 
             LIMIT 1)
  ) OR (
    id IN (SELECT id 
             FROM updates 
             WHERE name = 'bar' 
             ORDER BY version DESC 
             LIMIT 1)
  );

This works, but feels wrong. If I want to filter on more names, I have to replicate the whole subquery multiple times. Is there a better way to do this?

回答1:

select distinct on (name) id, name, version
from metadata
where name in ('foo', 'bar')
order by name, version desc

回答2:

NOT EXISTS is a way to avoid unwanted sub optimal tuples:

SELECT * 
FROM updates uu
WHERE uu.zname IN ('foo', 'bar')
AND NOT EXISTS (
    SELECT *
    FROM updates nx
    WHERE nx.zname = uu.zanme
    AND nx.version > uu.version
    );

Note: I replaced name by zname, since it is more or less a keyword in postgresql.

回答3:

Update after rereading the Q:

I want to select all rows where the name is one of some given list of names and the version is maximum of all the rows with that name.

If there can be ties (multiple rows with the maximum version per name), you could use the window function rank() in a subquery. Requires PostgreSQL 8.4+.

SELECT *
FROM  (
   SELECT *, rank() OVER (PARTITION BY name ORDER BY version DESC) AS rnk
   FROM   updates 
   WHERE  name IN ('foo', 'bar')
   )
WHERE rnk = 1;

来源：https://stackoverflow.com/questions/16119906/select-rows-satisfying-some-criteria-and-with-maximum-value-in-a-certain-column

标签

sql

postgresql

greatest-n-per-group