问题
I have two tables players and scores.
I want to generate a report that looks something like this:
player first score points
foo 2010-05-20 19
bar 2010-04-15 29
baz 2010-02-04 13
Right now, my query looks something like this:
select p.name player,
min(s.date) first_score,
s.points points
from players p
join scores s on s.player_id = p.id
group by p.name, s.points
I need the s.points that is associated with the row that min(s.date) returns. Is that happening with this query? That is, how can I be certain I'm getting the correct s.points value for the joined row?
Side note: I imagine this is somehow related to MySQL's lack of dense ranking. What's the best workaround here?
回答1:
This is the greatest-n-per-group problem that comes up frequently on Stack Overflow.
Here's my usual answer:
select
p.name player,
s.date first_score,
s.points points
from players p
join scores s
on s.player_id = p.id
left outer join scores s2
on s2.player_id = p.id
and s2.date < s.date
where
s2.player_id is null
;
In other words, given score s, try to find a score s2 for the same player, but with an earlier date. If no earlier score is found, then s is the earliest one.
Re your comment about ties: You have to have a policy for which one to use in case of a tie. One possibility is if you use auto-incrementing primary keys, the one with the least value is the earlier one. See the additional term in the outer join below:
select
p.name player,
s.date first_score,
s.points points
from players p
join scores s
on s.player_id = p.id
left outer join scores s2
on s2.player_id = p.id
and (s2.date < s.date or s2.date = s.date and s2.id < s.id)
where
s2.player_id is null
;
Basically you need to add tiebreaker terms until you get down to a column that's guaranteed to be unique, at least for the given player. The primary key of the table is often the best solution, but I've seen cases where another column was suitable.
Regarding the comments I shared with @OMG Ponies, remember that this type of query benefits hugely from the right index.
回答2:
Most RDMBs won't even let you include non aggregate columns in your SELECT clause when using GROUP BY. In MySQL, you'll end up with values from random rows for your non-aggregate columns. This is useful if you actually have the same value in a particular column for all the rows. Therefore, it's nice that MySQL doesn't restrict us, though it's an important thing to understand.
A whole chapter is devoted to this in SQL Antipatterns.
来源：https://stackoverflow.com/questions/3120423/join-single-row-from-a-table-in-mysql