Delete Cascade Multiple Tables - mysql

Question

I wonder whether someone can help me please.
I have the following three tables:
Parent Table
CREATE TABLE `userdetails` (
`userid` int(6) NOT NULL auto_increment,
`forename` varchar(20) NOT NULL,
`surname` varchar(30) NOT NULL,
`emailaddress` varchar(150) NOT NULL,
`password` varchar(200) NOT NULL,
`passwordhint` varchar(20) NOT NULL,
`subscriptionexpiration` date NOT NULL,
`salt` varchar(200) NOT NULL,
PRIMARY KEY (`userid`),
UNIQUE KEY `emailaddress` (`emailaddress`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
Child Table
CREATE TABLE `detectinglocations` (
`userid` int(6) NOT NULL,
`locationid` int(6) NOT NULL auto_increment,
`locationname` varchar(80) NOT NULL,
`address` varchar(110) NOT NULL,
`osgb36lat` float(10,6) NOT NULL,
`osgb36lon` float(10,6) NOT NULL,
`osgridref` varchar(20) NOT NULL,
`wgs84latd` int(2) NOT NULL,
`wgs84latm` int(2) NOT NULL,
`wgs84lats` decimal(6,2) NOT NULL,
`wgs84latb` varchar(1) NOT NULL,
`wgs84lond` int(2) NOT NULL,
`wgs84lonm` int(2) NOT NULL,
`wgs84lons` decimal(6,2) NOT NULL,
`wgs84lonb` varchar(1) NOT NULL,
`nameoflocationcontact` varchar(30) NOT NULL,
`locationcontactsaddressline1` varchar(50) NOT NULL,
`locationcontactsaddressline2` varchar(50) default NULL,
`locationcontactsaddressline3` varchar(50) default NULL,
`locationcontactsaddressline4` varchar(50) default NULL,
`locationcontactstelephonenumber` varchar(15) default NULL,
PRIMARY KEY (`locationid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
Child Table
CREATE TABLE `detectors` (
`userid` int(6) NOT NULL,
`detectorid` int(6) NOT NULL auto_increment,
`detectorname` varchar(30) NOT NULL,
PRIMARY KEY (`detectorid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 PACK_KEYS=0 AUTO_INCREMENT=1 ;
Using the this code below, I'm trying to implement the 'Delete Cascade' functionality, whereby if the user is deleted from the parent table, the associated rows within the child tables are alse deleted.
ALTER TABLE 'tablename'
add CONSTRAINT fk_userdetails
FOREIGN KEY (userid)
REFERENCES userdetails(userid)
ON DELETE CASCADE
I can successfully implement this for the first child table, but when I try to do the same with the second child table I receive the following error:
#1005 - Can't create table './db369054642/#sql-30d_bd1a57.frm' (errno: 121)
I've done quite a bit of research to find out what the problem may be, but I must admit I'm none the wiser.
Could someone perhaps have a look at this please and let me know what I'm doing wrong?
Many thanks

Answer 1

The solution is on http://dev.mysql.com/doc/refman/5.0/en/innodb-error-codes.html in the post by Mark Robbins on September 29 2007 10:19pm and the next one:
Constraint names must be unique
fk_userdetails is not unique
So you might try
ALTER TABLE 'tablename'
add CONSTRAINT fk_userdetails_detectors
FOREIGN KEY (userid)
REFERENCES userdetails(userid)
ON DELETE CASCADE
on the second table