Enforce unique values across two tables

Question

Is it possible to enforce uniqueness across two tables in MySQL?

I have two tables, both describing users. The users in these tables were for two different systems p

Answer 1

You could add an extra table with a single column as a primary key. Then create a trigger on each of your old user tables to insert the id into this extra table.

create table users1 (
    user_id integer primary key,
    username varchar(8) not null unique
);
create table users2 (
    user_id integer primary key,
    username varchar(8) not null unique
);
create table all_usernames (
    username varchar(8) primary key
);
create trigger users1_insert before insert on users1 for each row
    insert into all_usernames values(new.username);
create trigger users2_insert before insert on users2 for each row
    insert into all_usernames values(new.username);
create trigger users1_update before update on users1 for each row
    update all_usernames set username = new.username
    where username = old.username;
create trigger users2_update before update on users2 for each row
    update all_usernames set username = new.username
    where username = old.username;
create trigger users1_delete before delete on users1 for each row
    delete from all_usernames where username = old.username;
create trigger users2_delete before delete on users2 for each row
    delete from all_usernames where username = old.username;

You can then populate the table with

insert into all_usernames select username from users1;
insert into all_usernames select username from users2;