问题
I have table data which looks like this
id | keyword | count | date
1 | ipod | 200 | 2009-08-02
2 | ipod | 250 | 2009-09-01
3 | ipod | 150 | 2009-09-04
4 | ipod | NULL | 2009-09-07
Now what I am after is getting the count of the row which has the newest date but has a not null count. In which case row 3 with count 150.)
eg
SELECT `keyword`, `count` , max( `date` )
FROM `keywords`
WHERE keyword = "ipod"
AND `count` IS NOT NULL
GROUP BY keyword
This returned the right date but not the right count (returned 200). I also tried doing a left join on it's self.
SELECT `t1`.`keyword` , `t2`.`count` , max( `t1`.`id` )
FROM `keywords` `t1`
LEFT JOIN `keywords` `t2` ON `t1`.`id` = `t2`.`id`
WHERE `t1`.`keyword` = 'ipod'
AND `t1`.`count` IS NOT NULL
GROUP BY `t1`.`keyword`
And this did get the max id but it didn't left-join onto that like I hoped and returned only 200.
Help?
回答1:
SELECT `keyword`, `count` , `date`
FROM `keywords`
WHERE keyword = "ipod"
AND `count` IS NOT NULL
order by date desc
limit 1
回答2:
I'm sure there's a better way, but in the mean time:
SELECT `count`
FROM `keywords`
WHERE keyword = "ipod"
AND `count` IS NOT NULL
ORDER BY `date` DESC
LIMIT 1
Should fit your requirements. Note that LIMIT is not portable SQL, so it will only work for MySQL. The ORDER BY clause is necessary to force the database to sort by, well, date.
When I mentioned that there should be a better way, that's merely because of the way the LIMIT clause works, which is: The database has to fetch ALL rows that meet the query criteria, then simply truncates the result set to the desired amount specified by LIMIT.
回答3:
Does this work for you:
SELECT `keyword`, `count`, `date`
FROM `keywords`
WHERE keyword = "ipod"
AND `count` IS NOT NULL
ORDER BY DATE DESC
LIMIT 1
来源：https://stackoverflow.com/questions/1740199/select-the-newest-record-with-a-non-null-value-in-one-column