Select multiple fields from subquery - mysql

Question

I have the next query:
SELECT
a.Date,
(SELECT SUM(Used), SUM(Max) FROM Switch_Statistic b WHERE Date = (SELECT MAX(Date) FROM Switch_Statistic WHERE Switch_ID = b.Switch_ID AND Date <= a.Date))
FROM Switch_Statistic a
GROUP BY Date;
As you see I need to select SUM(Used), SUM(Max) from subquery. With CONCAT is not good solution!
Table schema:
ID --- Switch_ID --- Date --- Max --- Used
Some data:
1 641 2014-10-04 2 16
20 630 2014-10-04 1 7
24 634 2014-10-04 0 8
26 641 2014-10-06 2 16
32 641 2014-10-07 2 16
35 641 2014-10-08 3 16
39 641 2014-10-09 2 16
64 293 2014-10-10 1 22
...
557 38 2014-10-12 3 22
559 293 2014-10-12 1 22
563 294 2014-10-12 6 22
565 641 2014-10-12 2 16
What I need:
Example with CONCAT_WS
mysql> SELECT
a.Date,
(SELECT CONCAT_WS('/', SUM(Used), SUM(Max)) FROM Switch_Statistic b WHERE Date = (SELECT MAX(Date) FROM Switch_Statistic WHERE Switch_ID = b.Switch_ID AND Date <= a.Date)) AS Result
FROM Switch_Statistic a
GROUP BY Date;
+------------+----------+
| Date | Result |
+------------+----------+
| 2014-10-04 | 3/31 |
| 2014-10-06 | 3/31 |
| 2014-10-07 | 3/31 |
| 2014-10-08 | 4/31 |
| 2014-10-09 | 3/31 |
| 2014-10-10 | 249/1587 |
| 2014-10-11 | 354/2147 |
| 2014-10-12 | 360/2185 |
+------------+----------+
8 rows in set (0.26 sec)
Query logic:
1) Select all date's from table
2) SUM - Used and Max for current date, if Switch_ID don't have record for this date, then select the last which exists in table
Link to sqlfiddle - http://sqlfiddle.com/#!2/c3d479

Answer 1

You should be able to do this with just aggregation and no subqueries or joins:
SELECT date, sum(used) as used, sum(max) as max
FROM switch_statistic ss
where ss.date = (select max(date) from Switch_Statistics ss2 where ss2.Switch_id = ss.SwitchId
GROUP BY ss.date;
EDIT:
You seem to want a cumulative sum. In MySQL, this is often best done using variables:
SELECT date, used, max, (#u := #u + used) as cumeused, #m := #m + max) as cumemax
fROM (SELECT date, sum(used) as used, sum(max) as max
FROM switch_statistic ss
GROUP BY ss.date
) ss CROSS JOIN
(SELECT #u := 0, #m := 0) vars
ORDER BY date;