UNION with different columns

Hi all,

I have a problem on how to use UNION in MySQL i for two tables with different number of columns.

1st table (user)
user_id, name, position, manage, username and password

2nd table (client)
client_id, client_name, client_contact, occupation, email, member_type, username and password

How to do UNION for this two tables? I want to do UNION for these tables for log in purpose.

Thanks for the help :)