Cascade delete question

Hi All,

I am using a cascade delete on the following table:

vid    vname        parentname
1     n1@test.com
2     n2@test.com         n1@test.com
3     n3@test.com         n1@test.com
4     n4@test.com         n1@test.com
5     n5@test.com         n3@test.com

So the delete cascade states if I am deleting a row whose vname matches 
the parentname delete those rows and it works fine.

I just changed the values of parentname (but the vname values stay the same)

vid    vname        parentname
1     n1@test.com
2     n2@test.com         n1
3     n3@test.com         n1
4     n4@test.com         n1
5     n5@test.com         n3

Is there a way to do the same cascade delete with these values? I can 
select the correct info from vname in a query:

select substring(vname, from 0 for position('@' in vname)) from table1;

This works, so I tried to put that in the cascade but it failed.

Is there any way to accomplish this?
    Thanks
        Glenn MacGregor