＃1215-無法添加外鍵約束

Question

ALTER TABLE `attendance`
  ADD CONSTRAINT `attendance_ibfk_1` FOREIGN KEY (`subid`) REFERENCES `subject` (`subid`) ON DELETE CASCADE ON UPDATE CASCADE,
  ADD CONSTRAINT `attendance_ibfk_2` FOREIGN KEY (`studid`) REFERENCES `studentdetails` (`studid`) ON DELETE CASCADE ON UPDATE CASCADE;

--attendance table is

CREATE TABLE IF NOT EXISTS `attendance` (
  `attid` bigint(4) NOT NULL AUTO_INCREMENT,
  `studid` varchar(20) NOT NULL,
  `subid` bigint(4) NOT NULL,
  `totalclasses` int(2) NOT NULL,
  `attendedclasses` int(2) NOT NULL,
  `percentage` double(4,2) NOT NULL,
  `comment` text NOT NULL,
  PRIMARY KEY (`attid`),
  KEY `studid` (`studid`),
  KEY `subid` (`subid`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=7 ;

--subject table is...

CREATE TABLE IF NOT EXISTS `subject` (
  `subid` bigint(4) NOT NULL AUTO_INCREMENT,
  `subname` varchar(20) NOT NULL,
  `courseid` bigint(4) NOT NULL,
  `lecid` bigint(4) NOT NULL,
  `subtype` varchar(25) NOT NULL,
  `semester` varchar(25) NOT NULL,
  `comment` text NOT NULL,
  PRIMARY KEY (`subid`),
  KEY `courseid` (`courseid`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=8 ;

-- Table structure for table `studentdetails`
--

CREATE TABLE IF NOT EXISTS `studentdetails` (
  `studid` varchar(25) NOT NULL,
  `studfname` varchar(20) NOT NULL,
  `studlname` varchar(20) NOT NULL,
  `fathername` varchar(25) NOT NULL,
  `gender` varchar(20) NOT NULL,
  `address` varchar(100) NOT NULL,
  `contactno` varchar(20) NOT NULL,
  `courseid` bigint(4) NOT NULL,
  `semester` varchar(20) NOT NULL,
  `dob` date NOT NULL,
  PRIMARY KEY (`studid`),
  KEY `courseid` (`courseid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Answer 1

類型不匹配。 attendance.studid是varchar(20)但studentdetails.studid是varchar(25)

我猜兩者都應該是varchar(20)或varchar(25) 。