Using "OR" in MySql

I want to be able to marry up users from the aliases file and mailertable file using the username from the passwd file in Linux. However, the query doesn't seem to work. There is no problem taking data from one file using <BLOCKQUOTE><font size="1" face="Verdana,Arial,Helvetica">code:</font><HR><pre>WHERE passwd.username = aliases.username[/quote] but when I try that with two files, I can't get any data using <BLOCKQUOTE><font size="1" face="Verdana,Arial,Helvetica">code:</font><HR><pre>WHERE (passwd.username = aliases.username) OR (passwd.username = mailertable.username)[/quote]

Has anyone got any ideas.