Cannot delete foreign key constraint fails - mysql

Question

When I try to delete the question from the test, the sql show :
`Cannot delete or update a parent row: a foreign key constraint fails (`oes`.`studentquestion`,
CONSTRAINT `studentquestion_ibfk_2` FOREIGN KEY (`testid`, `qnid`) REFERENCES `question`
(`testid`, `qnid`))`
Here will be my table:
student table
CREATE TABLE `student` (
`stdid` bigint(20) NOT NULL,
`subid` int(11) NOT NULL,
`stdname` varchar(40) default NULL,
`stdpassword` varchar(40) default NULL,
`emailid` varchar(40) default NULL,
`contactno` varchar(20) default NULL,
`address` varchar(40) default NULL,
`city` varchar(40) default NULL,
`pincode` varchar(20) default NULL,
PRIMARY KEY (`stdid`),
UNIQUE KEY `stdname` (`stdname`),
UNIQUE KEY `emailid` (`emailid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
studentquestion table
CREATE TABLE `studentquestion` (
`stdid` bigint(20) NOT NULL default '0',
`testid` bigint(20) NOT NULL default '0',
`qnid` int(11) NOT NULL default '0',
`answered` enum('answered','unanswered','review') DEFAULT NULL,
`stdanswer` enum('optiona','optionb','optionc','optiond') DEFAULT NULL,
PRIMARY KEY (`stdid`,`testid`,`qnid`),
KEY `testid` (`testid`,`qnid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
studenttest table
`stdid` bigint(20) NOT NULL default '0',
`testid` bigint(20) NOT NULL default '0',
`starttime` timestamp NOT NULL default CURRENT_TIMESTAMP,
`endtime` timestamp NOT NULL default '0000-00-00 00:00:00',
`correctlyanswered` int(11) default NULL,
`status` enum('over','inprogress') default NULL,
PRIMARY KEY (`stdid`,`testid`),
KEY `testid` (`testid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
question table
CREATE TABLE `question` (
`testid` bigint(20) NOT NULL default '0',
`qnid` int(11) NOT NULL default '0',
`question` varchar(500) default NULL,
`optiona` varchar(100) DEFAULT NULL,
`optionb` varchar(100) DEFAULT NULL,
`optionc` varchar(100) DEFAULT NULL,
`optiond` varchar(100) DEFAULT NULL,
`correctanswer` enum ('optiona','optionb','optionc','optiond') DEFAULT NULL,
`marks` int(11) DEFAULT NULL,
PRIMARY KEY (`testid`,`qnid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
test table
CREATE TABLE `test` (
`testid` bigint(20) NOT NULL,
`testname` varchar(30) NOT NULL,
`testdesc` varchar(100) default NULL,
`testdate` date default NULL,
`testtime` time default NULL,
`subid` int(11) default NULL,
`testfrom` timestamp NOT NULL default CURRENT_TIMESTAMP,
`testto` timestamp NOT NULL default '0000-00-00 00:00:00',
`duration` int(11) default NULL,
`totalquestions` int(11) default NULL,
`attemptedstudents` bigint(20) DEFAULT NULL,
`testcode` varchar(40) NOT NULL,
`tcid` bigint(20) default NULL,
`minimunscore` int(11) NOT NULL,
PRIMARY KEY (`testid`),
UNIQUE KEY `testname` (`testname`),
KEY `test_fk1` (`subid`),
KEY `test_fk2` (`tcid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
subject table
CREATE TABLE `subject` (
`subid` int(11) NOT NULL,
`subname` varchar(40) default NULL,
`subdesc` varchar(100) default NULL,
`tcid` bigint(20) default NULL,
PRIMARY KEY (`subid`),
UNIQUE KEY `subname` (`subname`),
KEY `subject_fk1` (`tcid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
ALTER TABLE `studentquestion`
ADD CONSTRAINT `studentquestion_ibfk_1` FOREIGN KEY (`stdid`) REFERENCES `student`
(`stdid`),
ADD CONSTRAINT `studentquestion_ibfk_2` FOREIGN KEY (`testid`, `qnid`) REFERENCES `question`
(`testid`, `qnid`);
ALTER TABLE `studenttest`
ADD CONSTRAINT `studenttest_ibfk_1` FOREIGN KEY (`stdid`) REFERENCES `student` (`stdid`),
ADD CONSTRAINT `studenttest_ibfk_2` FOREIGN KEY (`testid`) REFERENCES `test` (`testid`);
ALTER TABLE `question`
ADD CONSTRAINT `question_ibfk_1` FOREIGN KEY (`testid`) REFERENCES `test` (`testid`);
ALTER TABLE `test`
ADD CONSTRAINT `test_fk1` FOREIGN KEY (`subid`) REFERENCES `subject` (`subid`),
ADD CONSTRAINT `test_fk2` FOREIGN KEY (`tcid`) REFERENCES `testconductor` (`tcid`);
ALTER TABLE `subject`
ADD CONSTRAINT `subject_fk1` FOREIGN KEY (`tcid`) REFERENCES `testconductor` (`tcid`);
INSERT INTO `adminlogin` VALUES ('001','root','root');
INSERT INTO `studenttest` (`stdid`, `testid`, `starttime`, `endtime`, `correctlyanswered`,
`status`) VALUES
(1, 1, '2014-10-15 09:11:24', '2014-10-15 09:21:24', 0, 'over');
INSERT INTO `subject` (`subid`, `subname`, `subdesc`, `tcid`) VALUES
(1, 'fref', 'few', NULL);
INSERT INTO `test` (`testid`, `testname`, `testdesc`, `testdate`, `testtime`, `subid`,
`testfrom`, `testto`, `duration`, `totalquestions`, `attemptedstudents`, `testcode`, `tcid`)
VALUES
(1, 'gregre', 'greger', '2014-10-15', '17:08:16', 1, '2014-10-15 03:08:16', '2014-10-16
15:59:59', 10, 2, 0, '.ȁ', NULL);
When the student has complete the test, then the question will not be able to delete. Neither the test or subject.

Answer 1

It is because the studentquestion table will be left with a foreign key that points to a non existent record. You'll either need to remove rows which have a foreign key to the record you want to delete or, alternatively set up CASCADE rules so that when parent rows are deleted action can be taken.