Question

Let us consider a user table:
Username,email1,email2

Now, I want to make sure that no two rows in this table have any common email address. What would be the best way to ensure this ?

E.g.: If table has row: “Bill,[e-mail],[e-mail]”, then
trying to insert a row like “Billy,[e-mail],[e-mail]” should give an error.

Thanks in advance,
Chandresh

Answer 1

Sadly, check constraints are not a feature of MySQL. Well, they are, but they’re ignored.

Use a trigger, instead:

create trigger MyValidation after insert on user for each row
begin
    if (select count(*) from user where 
        email1 = new.email1
        or email1 = new.email2 
        or email2 = new.email1 
        or email2 = new.email2) > 0 then
        --I know that the delete could use this logic, but you can
        --insert other instructions here.

        delete from user where username = new.username

    end if
end