I have this table for documents (simplified version here):
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?
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 Stack Overflow 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
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 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 a
LEFT JOIN, remember?). Then, we filter the joined result, showing only the rows where the right side is
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
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.
My preference is to use as little code as possible…
You can do it using
SELECT * FROM t1 WHERE (id,rev) IN ( SELECT id, MAX(rev) FROM t1 GROUP BY id )
to my mind it is less complicated… easier to read and maintain.
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.
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 can’t vouch for the performance, but here’s a trick inspired by the limitations of Microsoft Excel. It has some good features
- It should force return of only one “max record” even if there is a tie (sometimes useful)
- It doesn’t require a join
It is a little bit ugly and requires that you know something about the range of valid values of the rev column. Let us assume that we know the rev column is a number between 0.00 and 999 including decimals but that there will only ever be two digits to the right of the decimal point (e.g. 34.17 would be a valid value).
The gist of the thing is that you create a single synthetic column by string concatenating/packing the primary comparison field along with the data you want. In this way, you can force SQL’s MAX() aggregate function to return all of the data (because it has been packed into a single column). Then you have to unpack the data.
Here’s how it looks with the above example, written in SQL
SELECT id, CAST(SUBSTRING(max(packed_col) FROM 2 FOR 6) AS float) as max_rev, SUBSTRING(max(packed_col) FROM 11) AS content_for_max_rev FROM (SELECT id, CAST(1000 + rev + .001 as CHAR) || '---' || CAST(content AS char) AS packed_col FROM yourtable ) GROUP BY id
The packing begins by forcing the rev column to be a number of known character length regardless of the value of rev so that for example
- 3.2 becomes 1003.201
- 57 becomes 1057.001
- 923.88 becomes 1923.881
If you do it right, string comparison of two numbers should yield the same “max” as numeric comparison of the two numbers and it’s easy to convert back to the original number using the substring function (which is available in one form or another pretty much everywhere).
Unique Identifiers? Yes! Unique identifiers!
One of the best ways to develop a MySQL DB is to have each
AUTOINCREMENT (Source MySQL.com). This allows a variety of advantages, too many to cover here. The problem with the question is that its example has duplicate ids. This disregards these tremendous advantages of unique identifiers, and at the same time, is confusing to those familiar with this already.
Newer versions of MySQL come with
ONLY_FULL_GROUP_BY enabled by default, and many of the solutions here will fail in testing with this condition.
Even so, we can simply select
), etc., and have no worries understanding the result or how the query works :
SELECT DISTINCT t1.id, MAX(t1.rev), MAX(t2.content) FROM Table1 AS t1 JOIN Table1 AS t2 ON t2.id = t1.id AND t2.rev = ( SELECT MAX(rev) FROM Table1 t3 WHERE t3.id = t1.id ) GROUP BY t1.id;
SELECT DISTINCT Table1.id, max(Table1.rev), max(Table2.content): Return
MAX()some otherfield, the last
MAX()is redundant, because I know it’s just one row, but it’s required by the query.
FROM Employee: Table searched on.
JOIN Table1 AS Table2 ON Table2.rev = Table1.rev: Join the second table on the first, because, we need to get the max(table1.rev)’s comment.
GROUP BY Table1.id: Force the top-sorted, Salary row of each employee to be the returned result.
Note that since “content” was “…” in OP’s question, there’s no way to test that this works. So, I changed that to “..a”, “..b”, so, we can actually now see that the results are correct:
id max(Table1.rev) max(Table2.content) 1 3 ..d 2 1 ..b
Why is it clean?
MAX(), etc., all make wonderful use of MySQL indices. This will be faster. Or, it will be much faster, if you have indexing, and you compare it to a query that looks at all rows.
ONLY_FULL_GROUP_BY disabled, we can use still use
GROUP BY, but then we are only using it on the Salary, and not the id:
SELECT * FROM (SELECT * FROM Employee ORDER BY Salary DESC) AS employeesub GROUP BY employeesub.Salary;
SELECT *: Return all fields.
FROM Employee: Table searched on.
(SELECT *...)subquery : Return all people, sorted by Salary.
GROUP BY employeesub.Salary: Force the top-sorted, Salary row of each employee to be the returned result.
Note the Definition of a Relational Database: “Each row in a table has its own unique key.” This would mean that, in the question’s example, id would have to be unique, and in that case, we can just do :
SELECT * FROM Employee WHERE Employee.id = 12345 ORDER BY Employee.Salary DESC LIMIT 1
Hopefully this is a solution that solves the problem and helps everyone better understand what’s happening in the DB.
Something like this?
SELECT yourtable.id, rev, content FROM yourtable INNER JOIN ( SELECT id, max(rev) as maxrev FROM yourtable GROUP BY id ) AS child ON (yourtable.id = child.id) AND (yourtable.rev = maxrev)
Another manner to do the job is using
MAX() analytic function in OVER PARTITION clause
SELECT t.* FROM ( SELECT id ,rev ,contents ,MAX(rev) OVER (PARTITION BY id) as max_rev FROM YourTable ) t WHERE t.rev = t.max_rev
ROW_NUMBER() OVER PARTITION solution already documented in this post is
SELECT t.* FROM ( SELECT id ,rev ,contents ,ROW_NUMBER() OVER (PARTITION BY id ORDER BY rev DESC) rank FROM YourTable ) t WHERE t.rank = 1
This 2 SELECT work well on Oracle 10g.
MAX() solution runs certainly FASTER that
ROW_NUMBER() solution because
MAX() complexity is
ROW_NUMBER() complexity is at minimum
n represent the number of records in table !
I like to use a
NOT EXIST-based solution for this problem:
SELECT id, rev -- you can select other columns here FROM YourTable t WHERE NOT EXISTS ( SELECT * FROM YourTable t WHERE t.id = id AND rev > t.rev )
This will select all records with max value within the group and allows you to select other columns.
I think, You want this?
select * from docs where (id, rev) IN (select id, max(rev) as rev from docs group by id order by id)
SQL Fiddle :
SELECT * FROM Employee where Employee.Salary in (select max(salary) from Employee group by Employe_id) ORDER BY Employee.Salary
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 maybe. You’ll also run into the
group_concat_max_len limit quicker.
WITH 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
Since this is most popular question with regard to this problem, I’ll re-post another answer to it here as well:
It looks like there is simpler way to do this (but only in MySQL):
select * from (select * from mytable order by id, rev desc ) x group by id
Please credit answer of user Bohemian in this question for providing such a concise and elegant answer to this problem.
Edit: though this solution works for many people it may not be stable in the long run, since MySQL doesn’t guarantee that GROUP BY statement will return meaningful values for columns not in GROUP BY list. So use this solution at your own risk!
I would use this:
select t.* from test as t join (select max(rev) as rev from test group by id) as o on o.rev = t.rev
Subquery SELECT is not too eficient maybe, but in JOIN clause seems to be usable. I’m not an expert in optimizing queries, but I’ve tried at MySQL, PostgreSQL, FireBird and it does work very good.
You can use this schema in multiple joins and with WHERE clause. It is my working example (solving identical to yours problem with table “firmy”):
select * from platnosci as p join firmy as f on p.id_rel_firmy = f.id_rel join (select max(id_obj) as id_obj from firmy group by id_rel) as o on o.id_obj = f.id_obj and p.od > '2014-03-01'
It is asked on tables having teens thusands of records, and it takes less then 0,01 second on really not too strong machine.
I wouldn’t use IN clause (as it is mentioned somewhere above). IN is given to use with short lists of constans, and not as to be the query filter built on subquery. It is because subquery in IN is performed for every scanned record which can made query taking very loooong time.
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
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
This solution makes only one selection from YourTable, therefore it’s faster. It works only for MySQL and SQLite(for SQLite remove DESC) according to test on sqlfiddle.com. Maybe it can be tweaked to work on other languages which I am not familiar with.
SELECT * FROM ( SELECT * FROM ( SELECT 1 as id, 1 as rev, 'content1' as content UNION SELECT 2, 1, 'content2' UNION SELECT 1, 2, 'content3' UNION SELECT 1, 3, 'content4' ) as YourTable ORDER BY id, rev DESC ) as YourTable GROUP BY id
Here is a nice way of doing that
Use following code :
with temp as ( select count(field1) as summ , field1 from table_name group by field1 ) select * from temp where summ = (select max(summ) from temp)
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
Sorted the rev field in reverse order and then grouped by id which gave the first row of each grouping which is the one with the highest rev value.
SELECT * FROM (SELECT * FROM table1 ORDER BY id, rev DESC) X GROUP BY X.id;
Tested in http://sqlfiddle.com/ with the following data
CREATE TABLE table1 (`id` int, `rev` int, `content` varchar(11)); INSERT INTO table1 (`id`, `rev`, `content`) VALUES (1, 1, 'One-One'), (1, 2, 'One-Two'), (2, 1, 'Two-One'), (2, 2, 'Two-Two'), (3, 2, 'Three-Two'), (3, 1, 'Three-One'), (3, 3, 'Three-Three') ;
This gave the following result in MySql 5.5 and 5.6
id rev content 1 2 One-Two 2 2 Two-Two 3 3 Three-Two
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
None of these answers have worked for me.
This is what worked for me.
with score as (select max(score_up) from history) select history.* from score, history where history.score_up = score.max
Here’s another solution to retrieving the records only with a field that has the maximum value for that field. This works for SQL400 which is the platform I work on. In this example, the records with the maximum value in field FIELD5 will be retrieved by the following SQL statement.
SELECT A.KEYFIELD1, A.KEYFIELD2, A.FIELD3, A.FIELD4, A.FIELD5 FROM MYFILE A WHERE RRN(A) IN (SELECT RRN(B) FROM MYFILE B WHERE B.KEYFIELD1 = A.KEYFIELD1 AND B.KEYFIELD2 = A.KEYFIELD2 ORDER BY B.FIELD5 DESC FETCH FIRST ROW ONLY)
This is not pure SQL. This will use the SQLAlchemy ORM.
I came here looking for SQLAlchemy help, so I will duplicate Adrian Carneiro’s answer with the python/SQLAlchemy version, specifically the outer join part.
This query answers the question of:
“Can you return me the records in this group of records (based on same id) that have the highest version number”.
This allows me to duplicate the record, update it, increment its version number, and have the copy of the old version in such a way that I can show change over time.
MyTableAlias = aliased(MyTable) newest_records = appdb.session.query(MyTable).select_from(join( MyTable, MyTableAlias, onclause=and_( MyTable.id == MyTableAlias.id, MyTable.version_int < MyTableAlias.version_int ), isouter=True ) ).filter( MyTableAlias.id == None, ).all()
Tested on a PostgreSQL database.
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
You can make the select without a join when you combine the
id into one
maxRevId value for
MAX() and then split it back to original values:
SELECT maxRevId & ((1 << 32) - 1) as id, maxRevId >> 32 AS rev FROM (SELECT MAX(((rev << 32) | id)) AS maxRevId FROM YourTable GROUP BY id) x;
This is especially fast when there is a complex join instead of a single table. With the traditional approaches the complex join would be done twice.
The above combination is simple with bit functions when
INT UNSIGNED (32 bit) and combined value fits to
BIGINT UNSIGNED (64 bit). When the
rev are larger than 32-bit values or made of multiple columns, you need combine the value into e.g. a binary value with suitable padding for