I use
- Linux
- mysql 5.0.18
- mysql-connector-odbc-3.51.20-0
- the tables are create with this SQL command:
drop TABLE if EXISTS rel1 ;
CREATE TABLE `rel1` (
  `id` int(11) NOT NULL,
  `text_1` varchar(20) default NULL,
  `text_2` varchar(20) default NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1; 

drop TABLE if EXISTS rel2;
CREATE TABLE `rel2` (
  `id` int(11) NOT NULL,
  `text_1` varchar(20) default NULL,
  `text_2` varchar(20) default NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1; 

1. open a database with type mysql and ODBC to connect to the mysql server 
2. open TOOLS / RELATIONS
3. add table rel1 and rel2
4. d&d id field from rel1 to rel2
5. right click the relation
6. choose delete
==>> error messages: You have an error in your SQL syntax: 'CONSTRAINT 'NULL''