一个关于MYSQL IFNULL的用法

select a.receiveID,(a.num - IFNULL(b.num,0)) as num 
from (SELECT num,receiveID from dog_giftnumrecord where giftID = 1) as a 
left join (SELECT num,receiveID from dog_giftnumrecord where giftID = 2 ) as b 
on a.receiveID = b.receiveID

--
-- 表的结构 `dog_giftnumrecord`
--

CREATE TABLE IF NOT EXISTS `dog_giftnumrecord` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT COMMENT '道具计数表ID',
  `giftID` int(11) NOT NULL COMMENT '道具ID',
  `receiveID` int(11) NOT NULL COMMENT '接受者ID',
  `type` int(11) NOT NULL COMMENT '道具对应类型(1对应会员,2对应狗狗)',
  `num` int(11) NOT NULL COMMENT '道具数量',
  `createDate` int(11) NOT NULL COMMENT '创建时间',
  `updateDate` int(11) DEFAULT NULL COMMENT '更新时间',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=11 ;

--
-- 转存表中的数据 `dog_giftnumrecord`
--

INSERT INTO `dog_giftnumrecord` (`id`, `giftID`, `receiveID`, `type`, `num`, `createDate`, `updateDate`) VALUES
(3, 1, 138, 1, 4, 1413014344, 1413015921),
(4, 1, 139, 1, 2, 1413015942, 1413015944),
(5, 2, 139, 1, 1, 1413015946, NULL),
(6, 1, 140, 1, 3, 1413015961, 1413015963),
(7, 2, 140, 1, 2, 1413015965, 1413015967),
(8, 1, 141, 1, 3, 1413015972, 1413015973),
(9, 1, 142, 1, 1, 1413015978, NULL),
(10, 2, 142, 1, 8, 1413015980, 1413018250);