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General Tech Bugs & Fixes 2 years ago
Posted on 16 Aug 2022, this text provides information on Bugs & Fixes related to General Tech. Please note that while accuracy is prioritized, the data presented might not be entirely correct or up-to-date. This information is offered for general knowledge and informational purposes only, and should not be considered as a substitute for professional advice.
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This question already has an answer here:
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.
[1, 3, ...]
[2, 1, ..]
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?
while
UpdateAs the answers suggest, there is a SQL solution, and here a sqlfiddle demo.
Update 2I 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.
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.
id in (1,2,3)
One final note, as leftjoin creates n*(n+1)/2 joins in groups, its performance can be heavily affected by the size of groups...
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