Can I use non-aggregate columns with group by?

问题:

You cannot (should not) put non-aggregates in the SELECT line of a GROUP BY query.

I would however like access the one of the non-aggregates associated with the max. In plain english, I want a table with the oldest id of each kind.

CREATE TABLE stuff (
   id int,
   kind int,
   age int
);

This query gives me the information I'm after:

SELECT kind, MAX(age)
FROM stuff
GROUP BY kind;

But it's not in the most useful form. I really want the id associated with each row so I can use it in later queries.

I'm looking for something like this:

SELECT id, kind, MAX(age)
FROM stuff
GROUP BY kind;

That outputs this:

SELECT stuff.*
FROM
   stuff,
   ( SELECT kind, MAX(age)
     FROM stuff
     GROUP BY kind) maxes
WHERE
   stuff.kind = maxes.kind AND
   stuff.age = maxes.age

It really seems like there should be away to get this information without needing to join. I just need the SQL engine to remember the other columns when it's calculating the max.

回答1:

You can't get the Id of the row that MAX found, because there might not be only one id with the maximum age.

回答2:

You cannot (should not) put non-aggregates in the SELECT line of a GROUP BY query.

You can, and have to, define what you are grouping by for the aggregate function to return the correct result.

MySQL (and SQLite) decided in their infinite wisdom that they would go against spec, and allow queries to accept GROUP BY clauses missing columns quoted in the SELECT - it effectively makes these queries not portable.

It really seems like there should be away to get this information without needing to join.

Without access to the analytic/ranking/windowing functions that MySQL doesn't support, the self join to a derived table/inline view is the most portable means of getting the result you desire.

回答3:

I think it's tempting indeed to ask the system to solve the problem in one pass rather than having to do the job twice (find the max, and the find the corresponding id). You can do using CONCAT (as suggested in Naktibalda refered article), not sure that would be more effeciant

SELECT MAX( CONCAT( LPAD(age, 10, '0'), '-', id)
FROM STUFF1
GROUP BY kind;

Should work, you have to split the answer to get the age and the id. (That's really ugly though)

回答4:

In recent databases you can use sum() over (parition by ...) to solve this problem:

select id, kind, age as max_age from (
  select id, kind, age, max(age) over (partition by kind) as mage
    from table)
where age = mage

This can then be single pass

回答5:

You have to have a join because the aggregate function max retrieves many rows and chooses the max. So you need a join to choose the one that the agregate function has found.

To put it a different way how would you expect the query to behave if you replaced max with sum?

An inner join might be more efficient than your sub query though.

回答6:

PostgesSQL's DISTINCT ON will be useful here.

SELECT DISTINCT ON (kind) kind, id, age 
FROM stuff
ORDER BY kind, age DESC;

This groups by kind and returns the first row in the ordered format. As we have ordered by age in descending order, we will get the row with max age for kind.

P.S. columns in DISTINCT ON should appear first in order by