I have this table for documents (simplified version here):
+------+-------+--------------------------------------+
| id | rev | content |
+------+-------+--------------------------------------+
| 1 | 1 | ... |
| 2 | 1 | ... |
| 1 | 2 | ... |
| 1 | 3 | ... |
+------+-------+--------------------------------------+
How do I select one row per id and only the greatest rev?
With the above data, the result should contain two rows: [1, 3, ...]
and [2, 1, ..]
. I'm using MySQL.
Currently I use checks in the while
loop to detect and over-write old revs from the resultset. But is this the only method to achieve the result? Isn't there a SQL solution?
Update
As the answers suggest, there is a SQL solution, and here a sqlfiddle demo.
Update 2
I noticed after adding the above sqlfiddle, the rate at which the question is upvoted has surpassed the upvote rate of the answers. That has not been the intention! The fiddle is based on the answers, especially the accepted answer.
All you need is a GROUP BY
clause with the MAX
aggregate function:
SELECT id, MAX(rev)
FROM YourTable
GROUP BY id
I just noticed you need the content
column as well.
This is a very common question in SQL: find the whole data for the row with some max value in a column per some group identifier. I heard that a lot during my career. Actually, it was one the questions I answered in my current job's technical interview.
It is, actually, so common that StackOverflow community has created a single tag just to deal with questions like that: greatest-n-per-group.
Basically, you have two approaches to solve that problem:
group-identifier, max-value-in-group
Sub-queryIn this approach, you first find the group-identifier, max-value-in-group
(already solved above) in a sub-query. Then you join your table to the sub-query with equality on both group-identifier
and max-value-in-group
:
SELECT a.id, a.rev, a.contents
FROM YourTable a
INNER JOIN (
SELECT id, MAX(rev) rev
FROM YourTable
GROUP BY id
) b ON a.id = b.id AND a.rev = b.rev
In this approach, you left join the table with itself. Equality goes in the group-identifier
. Then, 2 smart moves:
NULL
in the right side (it's a LEFT JOIN
, remember?). Then, we filter the joined result, showing only the rows where the right side is NULL
.So you end up with:
SELECT a.*
FROM YourTable a
LEFT OUTER JOIN YourTable b
ON a.id = b.id AND a.rev < b.rev
WHERE b.id IS NULL;
Both approaches bring the exact same result.
If you have two rows with max-value-in-group
for group-identifier
, both rows will be in the result in both approaches.
Both approaches are SQL ANSI compatible, thus, will work with your favorite RDBMS, regardless of its "flavor".
Both approaches are also performance friendly, however your mileage may vary (RDBMS, DB Structure, Indexes, etc.). So when you pick one approach over the other, benchmark. And make sure you pick the one which make most of sense to you.