选择每个月的最后一天？

Question

I have the following data in MySQL:

+------------+-------+
| myDATE     | delta |
+------------+-------+
| 2015-08-29 |    12 |
| 2015-08-30 |    12 |
| 2015-08-31 |    10 |
| 2015-09-01 |     0 |
| 2015-09-02 |  0.15 |
+------------+-------+

I want to run a query that will select the last day of each month and show the data. I thought I had it working but its not showing yesterdays data.

select mydate, delta from data group by date_format(mydate, '%Y-%m');

Results in:

+------------+-------+
| mydate     | delta |
+------------+-------+
| 2015-08-29 |    12 |
| 2015-09-01 |     0 |
+------------+-------+

What am I missing?

I'm looking for it to return this: (ie. The last day of each month on record)

+------------+-------+
| myDATE     | delta |
+------------+-------+
| 2015-08-31 |    10 |
| 2015-09-02 |  0.15 |
+------------+-------+

Answer 1

select last_day(mydate), sum(delta) from data group by last_day(mydate)

last_day is a function which returns the last day of the month the given date

https://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html#function_last-day

to answer the updated question

select data.`myDate`, data.delta from data inner join
(select max(`myDate`) myDate from data group by date_format(mydate, '%Y-%m')) a
on a.myDate = data.myDate