MySQL : selecting the X smallest values
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13-09-2019 - |
Question
Let be a table like this :
CREATE TABLE `amoreAgentTST01` (
`moname` char(64) NOT NULL DEFAULT '',
`updatetime` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`data` longblob,
PRIMARY KEY (`moname`,`updatetime`)
I have a query to find the oldest records for each distinct 'moname', but only if there are multiple records for this 'moname' :
SELECT moname, updatetime FROM amoreAgentTST01 a
WHERE (SELECT count(*) FROM amoreAgentTST01 x WHERE x.moname = a.moname) > 1
AND a.updatetime = (SELECT min(updatetime) FROM amoreAgentTST01 y WHERE y.moname = a.moname) ;
My question is : how to do the same but selecting the X oldest values ? I now simply run this, delete the oldest values and rerun it... which is not so nice.
Seconds question is : what do you think of the above query ? can it be improved ? is there any obvious bad practice ?
Thank you in advance for your advices and help.
Barth
Solution
Would something like this work (untested):
SELECT moname, MIN(updatetime) FROM amoreAgentTST01
GROUP BY moname HAVING COUNT(moname)>1
Edit - the above is meant only as a replacement for your existing code, so it doesn't directly answer your question.
I think something like this should work for your main question:
SELECT moname, updatetime FROM amoreAgentTST01
GROUP BY moname, updatetime
HAVING COUNT(moname)>1
ORDER BY updatetime LIMIT 0, 10
Edit - sorry, the above won't work because it's returning only 10 records for all the monames - rather than the 10 oldest for each. Let me have a think.
One more go at this (admittedly, this one looks a bit convoluted):
SELECT a.moname, a.updatetime FROM amoreAgentTST01 a
WHERE EXISTS
(SELECT * FROM amoreAgentTST01 b
WHERE a.moname = b.moname AND a.updatetime = b.updatetime
ORDER BY b.updatetime LIMIT 0, 10)
AND (SELECT COUNT(*) FROM amoreAgentTST01 x WHERE x.moname = a.moname) > 1
I should add that if there is an ID column - generally the primary key- then that should be used for the sub-query joins for improved performance.