Free TimeSlot返回空数组

Question

这是一个创建我的表的查询：

CREATE TABLE IF NOT EXISTS `appschedule` (
  `PID` varchar(200) NOT NULL,
  `Surgery` varchar(200) NOT NULL,
  `Activity` varchar(200) NOT NULL,
  `Duration` int(11) NOT NULL,
  `AppDate` datetime NOT NULL,
  `AppSlot` varchar(20) NOT NULL,
  `TimeFrom` varchar(200) NOT NULL,
  `TimeTo` varchar(200) NOT NULL,
  `StartTime` datetime NOT NULL,
  `EndTime` datetime NOT NULL,
  `AppTag` varchar(50) NOT NULL,
  `Status` varchar(50) NOT NULL,
  `AppID` varchar(200) NOT NULL,
  `AppDateNum` varchar(20) NOT NULL,
  `AppNum` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

这里是插入查询

INSERT INTO `appschedule` 
(`PID`, `Surgery`, `Activity`, `Duration`, `AppDate`, `AppSlot`, `TimeFrom`, 
 `TimeTo`,    `StartTime`, `EndTime`, `AppTag`, `Status`, `AppID`, `AppDateNum`, 
`AppNum`) 
VALUES
('PID-2014-06-02-00001', 'Single', 'Extraction Of Incisors', 10, '2014-07-03
  00:00:00', 
 'Old', '17:30', '17:41', '2014-07-03 17:30:00', '2014-07-03 17:41:00', 
 'Single', '0', 'APP-2014-07-03-00001', '2014-07-03', 1);

现在我希望从上面的时间表中获得免费的时间段

所以我把这个查询

SELECT AvailStartTime, AvailEndTime 
    FROM 
        ( SELECT @lastEndTime as AvailStartTime, 
          StartTime as AvailEndTime, 
          @lastEndTime := EndTime 
           FROM (SELECT StartTime, EndTime 
                 FROM appschedule 
                  WHERE AppDateNum = '2014-07-03' 
                  AND AppDateNum = '2014-07-03' 
                  AND EndTime >= '2014-07-03 17:30' 
                  AND StartTime < '2014-07-03 21:30' 
                  ORDER BY StartTime) e JOIN 
                 (SELECT @lastEndTime := NULL) init) x 
                 WHERE AvailEndTime > DATE_ADD(AvailStartTime, INTERVAL 10 MINUTE) 

但我不知道为什么，它会给我留空数据。