How can I subtract two values selected from a table

Question

I have a MySQL table, like this:

CREATE TABLE IF NOT EXISTS `ladderStandard` (
  `charId` mediumint(8) unsigned NOT NULL,
  `poeRank` smallint(5) unsigned NOT NULL,
  `lvl` tinyint(3) unsigned NOT NULL,
  `exp` int(10) unsigned NOT NULL,
  `ladderTime` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00'
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;


INSERT INTO `ladderStandard` (`charId`, `poeRank`, `lvl`, `exp`, `ladderTime`) VALUES
(10000, 10000, 91, 2108226870, '2015-02-06 23:37:11'),
(10001, 10001, 91, 2108221545, '2015-02-06 23:37:11'),
(10002, 10002, 91, 2108219833, '2015-02-06 23:37:11'),
(10003, 10003, 91, 2108192924, '2015-02-06 23:37:11'),
(10004, 10004, 91, 2108154502, '2015-02-06 23:37:11'),
(10005, 10005, 91, 2108153763, '2015-02-06 23:37:11'),
(10000, 9998, 91, 2108226870, '2015-02-06 23:58:21'),
(10001, 9999, 91, 2108221545, '2015-02-06 23:58:21'),
(10002, 10000, 91, 2108219833, '2015-02-06 23:58:21'),
(10003, 10001, 91, 2108192924, '2015-02-06 23:58:21'),
(10004, 10002, 91, 2108154502, '2015-02-06 23:58:21'),
(10005, 10003, 91, 2108153763, '2015-02-06 23:58:21'),

I have two queries:

SELECT charId, exp FROM ladderStandard WHERE ladderTime = '2015-02-06 23:37:11';
SELECT charId, exp FROM ladderStandard WHERE ladderTime = '2015-02-06 23:58:21';

Now I want subtract these two queries to get charId and exp.from.first.query - exp.from.second.query

Here is an example on sqlfiddle.com .

Answer 1

You need to join both the queries then do the math

SELECT a.charid,
       a.exp - b.exp
FROM   (SELECT charId,
               exp
        FROM   ladderStandard
        WHERE  ladderTime = '2015-02-06 23:37:11') a
       JOIN (SELECT charId,
                    exp
             FROM   ladderStandard
             WHERE  ladderTime = '2015-02-06 23:58:21') b
         ON a.charid = b.charid 

SqlFiddle Demo

Answer 2

Use a self-JOIN. The WHERE clause will restrict each subset of the table to the specific time, and the ON clause links the corresponding rows.

SELECT a.charid, a.exp - b.exp AS diff
FROM ladderStandard AS a
JOIN ladderStandard AS b ON a.charid = b.charid
WHERE a.ladderTime = '2015-02-06 23:37:11'
AND b.ladderTime = '2015-02-06 23:58:21'

DEMO

Answer 3

In your sample data, the exp values are the same for the same ID in both groups. So subtracting one from the other will just result in zero.

However, on the assumption that your actual data has different values, here is a way to perform the operation without joining the table.

SELECT  CharId,
        Sum( case LadderTime
             when :SecondDate then exp
             else -exp end )as SumExp
FROM    LadderStandard
where   LadderTime in( :FirstDate, :SecondDate )
group by CharID;

As expected, this SQL Fiddle shows all zeros except for CharID = 10291, which has an entry in the second group but not the first. In that case, the calculation is 0 - exp.from.second.query which is just the negative of the exp value.