Mysql – How to selecting data where the date is with the next month or later

SELECT widget, MAX(`timestamp`) AS ts
FROM tableX AS t
WHERE state = 'down'
GROUP BY widget
HAVING NOT EXISTS
       ( SELECT *
         FROM tableX AS tt
         WHERE tt.widget = t.widget
           AND tt.state <> 'down'
           AND tt.`timestamp` > MAX(t.`timestamp`)
       ) ;

I think that you'll need two indices, one on (widget, state, timestamp) and one on (widget, timestamp, state) for efficiency.

This will work, too, and will be needing only one index, on (widget, timestamp, state):

SELECT t.widget, t.`timestamp`
FROM 
        tableX AS t
    JOIN
        ( SELECT widget, MAX(`timestamp`) AS ts
          FROM tableX
          GROUP BY widget
        ) AS tm
            ON  tm.widget = t.widget
            AND tm.ts = t.`timestamp`
WHERE t.state = 'down' ;

Tested both at SQL-Fiddle: test

You will want to use the DATE_FORMAT function in MySQL to format the date for each query.

-- query 1
select date_format(date, '%m-%d') `m-d`,
  sum(amount) Total
from mytable
group by date_format(date, '%m-%d');

See SQL Fiddle with Demo. Gives a result:

|   M-D | TOTAL |
-----------------
| 04-08 |    50 |
| 05-01 |   200 |
| 05-03 |   400 |

-- query 2
select date_format(date, '%Y-%m') `Y-m`,
  sum(amount) Total
from mytable
group by date_format(date, '%Y-%m');

See SQL Fiddle with Demo. Gives the result:

|     Y-M | TOTAL |
-------------------
| 2013-04 |    50 |
| 2013-05 |   600 |