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:
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?Answers
At first glance...
All you need is a
GROUP BY clause with the MAX aggregate function:SELECT id, MAX(rev)
FROM YourTable
GROUP BY id
It's never that simple, is it?
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 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:
Joining with simple group-identifier, max-value-in-group Sub-query
In 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
Left Joining with self, tweaking join conditions and filters
In this approach, you left join the table with itself. Equality, of course, goes in the
group-identifier. Then, 2 smart moves:- The second join condition is having left side value less than right value
- When you do step 1, the row(s) that actually have the max value will have
NULLin the right side (it's aLEFT JOIN, remember?). Then, we filter the joined result, showing only the rows where the right side isNULL.
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;
Conclusion
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.
Yet another solution is to use a correlated subquery:
select yt.id, yt.rev, yt.contents
from YourTable yt
where rev =
(select max(rev) from YourTable st where yt.id=st.id)
Having an index on (id,rev) renders the subquery almost as a simple lookup...
Following are comparisons to the solutions in @AdrianCarneiro's answer (subquery, leftjoin), based on MySQL measurements with InnoDB table of ~1million records, group size being: 1-3.
While for full table scans subquery/leftjoin/correlated timings relate to each other as 6/8/9, when it comes to direct lookups or batch (
id in (1,2,3)), subquery is much slower then the others (Due to rerunning the subquery). However I couldnt differentiate between leftjoin and correlated solutions in speed.
One final note, as leftjoin creates n*(n+1)/2 joins in groups, its performance can be heavily affected by the size of groups...
I am flabbergasted that no answer offered SQL window function solution:
SELECT a.id, a.rev, a.contents
FROM (SELECT id, rev, contents,
ROW_NUMBER() OVER (PARTITION BY id ORDER BY rev DESC) rank
FROM YourTable) a
WHERE a.rank = 1
Added in SQL standard ANSI/ISO Standard SQL:2003 and later extended with ANSI/ISO Standard SQL:2008, window (or windowing) functions are available with all major vendors now. There are more types of rank functions available to deal with a tie issue:
RANK, DENSE_RANK, PERSENT_RANK.
Something like this?
SELECT yourtable.id, rev, content
FROM yourtable
INNER JOIN (
SELECT id, max(rev) as maxrev FROM yourtable
WHERE yourtable
GROUP BY id
) AS child ON (yourtable.id = child.id) AND (yourtable.rev = maxrev)
A third solution I hardly ever see mentioned is MySQL specific and looks like this:
SELECT id, MAX(rev) AS rev
, 0+SUBSTRING_INDEX(GROUP_CONCAT(numeric_content ORDER BY rev DESC), ',', 1) AS numeric_content
FROM t1
GROUP BY id
Yes it looks awful (converting to string and back etc.) but in my experience it's usually faster than the other solutions. Maybe that just for my use cases, but I have used it on tables with millions of records and many unique ids. Maybe it's because MySQL is pretty bad at optimizing the other solutions (at least in the 5.0 days when I came up with this solution).
One important thing is that GROUP_CONCAT has a maximum length for the string it can build up. You probably want to raise this limit by setting the
group_concat_max_len variable. And keep in mind that this will be a limit on scaling if you have a large number of rows.
Anyway, the above doesn't directly work if your content field is already text. In that case you probably want to use a different separator, like \0 maybe. You'll also run into the
group_concat_max_len limit quicker.
How about this:
select all_fields.*
from (select id, MAX(rev) from yourtable group by id) as max_recs
left outer join yourtable as all_fields
on max_recs.id = all_fields.id
If you have many fields in select statement and you want latest value for all of those fields through optimized code:
select * from
(select * from table_name
order by id,rev desc) temp
group by id
NOT mySQL, but for other people finding this question and using SQL, another way to resolve the greatest-n-per-group problem is using
Cross Apply in MS SQLWITH DocIds AS (SELECT DISTINCT id FROM docs)
SELECT d2.id, d2.rev, d2.content
FROM DocIds d1
CROSS APPLY (
SELECT Top 1 * FROM docs d
WHERE d.id = d1.id
ORDER BY rev DESC
) d2
I like to do this by ranking the records by some column. In this case, rank
rev values grouped by id. Those with higher rev will have lower rankings. So highest rev will have ranking of 1.select id, rev, content
from
(select
@rowNum := if(@prevValue = id, @rowNum+1, 1) as row_num,
id, rev, content,
@prevValue := id
from
(select id, rev, content from YOURTABLE order by id asc, rev desc) TEMP,
(select @rowNum := 1 from DUAL) X,
(select @prevValue := -1 from DUAL) Y) TEMP
where row_num = 1;
Not sure if introducing variables makes the whole thing slower. But at least I'm not querying
YOURTABLE twice.
here is another solution hope it will help someone
Select a.id , a.rev, a.content from Table1 a
inner join
(SELECT id, max(rev) rev FROM Table1 GROUP BY id) x on x.id =a.id and x.rev =a.rev
SELECT * FROM Employee where Employee.Salary in (select max(salary) from Employee group by Employe_id) ORDER BY Employee.Salary
Many, if not all, of the other answers here are fine for small datasets. For scaling, more care is needed. See here.
It discusses multiple faster ways to do groupwise max and top-N per group.
I used the below to solve a problem of my own. I first created a temp table and inserted the max rev value per unique id.
CREATE TABLE #temp1
(
id varchar(20)
, rev int
)
INSERT INTO #temp1
SELECT a.id, MAX(a.rev) as rev
FROM
(
SELECT id, content, SUM(rev) as rev
FROM YourTable
GROUP BY id, content
) as a
GROUP BY a.id
ORDER BY a.id
I then joined these max values (#temp1) to all of the possible id/content combinations. By doing this, I naturally filter out the non-maximum id/content combinations, and am left with the only max rev values for each.
SELECT a.id, a.rev, content
FROM #temp1 as a
LEFT JOIN
(
SELECT id, content, SUM(rev) as rev
FROM YourTable
GROUP BY id, content
) as b on a.id = b.id and a.rev = b.rev
GROUP BY a.id, a.rev, b.content
ORDER BY a.id
select * from yourtable
group by id
having rev=max(rev);
SELECT * FROM t1 ORDER BY rev DESC LIMIT 1;
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