MySQL：同一日期的多行仅获取一行？

Question

I have a MySQL query and it returns several rows with a date in one column, thats my query:

select * from (big subselect) sub

I get 6 rows for example and one column in those 6 rows has a date, the column is called my_date. Is there a way to get only those rows that have a date that is not the same day? Example:

29.10.2012 01:51:10
29.10.2012 01:51:10
29.10.2012 01:51:10
29.10.2012 02:43:18
29.10.2012 02:43:18
29.10.2012 02:43:18

--> I want only one of the rows, because all 6 are the same day.

29.10.2012 01:51:10
30.10.2012 01:51:10 --> different
29.10.2012 01:51:10
29.10.2012 02:43:18
29.10.2012 02:43:18
29.10.2012 02:43:18

--> I want 2 rows, because there are 2 different days

The my_date column is not the only one in each row, but I want it as criteria. How could I do that?

Thanks!

Answer 1

You will need to select only these MAX(my_date) , but you will need to group by some other filed. Something like this:

select s1.*
from sub as s1
INNER JOIN
(
   SELECT MAX(my_date) AS MaxDate, SomeId
   FROM sub
   GROUP BY someId
) AS s2 ON s1.my_date = MaxDate AND s1.SomeId = s2.SomeId;

Answer 2

You can group only dates like this:

SELECT * FROM sub
GROUP BY DATE_FORMAT(my_date,'%Y-%m-%d')

See this SQLFiddle

Result:

| ID |                        MY_DATE |
---------------------------------------
|  1 | October, 29 2012 01:51:10+0000 |
|  2 | October, 30 2012 01:51:10+0000 |