Peter,
Thank you for your reply.
MAX(t1.occurrence ) will pull the max of the occurrence column out of
the group, but the other collumns (like data3 or id) would still be
sorted by the GROUP BY.
I will try your second solution, but the tables I am working are
thousands of row and your solution look to me like it would be
expensive as heck.
I am also considering creating a new table that would hold the values
of the most recent entries. It would something like:
CREATE TABLE `t2` (
`id` int(11) ,
`data1` varchar(50) ,
`data2` varchar(50) ,
UNIQUE KEY (`data1`,`data2`)
)
which I would keep up to date with triggers and join against the
previously mentioned t1.
On 9/25/07, Peter Brawley [EMAIL PROTECTED] wrote:
You might like to compare the performance of ...
SELECT t1.data1, t1.data2, MAX(t1.occurrence)
FROM t1
GROUP BY data1,data1
ORDER BY occurrence;
with...
SELECT t1.data1, t1.data2,t1.occurrence
FROM t1
LEFT JOIN t1 AS t2 ON t1.data1=t2.data2 AND t1.data2=t2.data2 AND
t1.occurrence t2.occurrence
WHERE t2.id IS NULL
ORDER BY occurrence;
PB
-
Rob Wultsch wrote:
Suppose I have a table:
CREATE TABLE `t1` (
`id` int(11) NOT NULL auto_increment,
`data1` varchar(50) ,
`data2` varchar(50) ,
`data3` varchar(50) ,
`occurance` datetime ,
PRIMARY KEY (`id`)
)
And I want to pull the most recent entry of each set of unique
combinations of `data1` and `data2`. Per the mysql manual:
If you use GROUP BY, output rows are sorted according to the GROUP BY
columns as if you had an ORDER BY for the same columns. To
avoid the overhead of sorting that GROUP BY produces, add
ORDER BY NULL:
What is the mysql syntax (if such syntax exists) to order within a
group by another field, in this example `occurance`?
Is there a better solution to this using sub selects?
Am I on crack for considering doing something like:
CREATE TEMPORARY TABLE `tmpt1` (
`id` int(11) NOT NULL auto_increment,
`data1` varchar(50) ,
`data2` varchar(50) ,
`data3` varchar(50) ,
`occurance` datetime ,
PRIMARY KEY (`id`),
UNIQUE(`data1`,`data2`)
) ;
INSERT IGNORE INTO `tmpt1`(
`id`,
`data1`,
`data2`,
`data3`,
`occurance`
) SELECT *
FROM `t1`
ORDER BY `occurance` DESC
--
Rob Wultsch
(480)223-2566
[EMAIL PROTECTED] (email/google im)
wultsch (aim)
[EMAIL PROTECTED] (msn)
--
Rob Wultsch
(480)223-2566
[EMAIL PROTECTED] (email/google im)
wultsch (aim)
[EMAIL PROTECTED] (msn)
--
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