Select data from another table where IDs donot match
<p>With the web application am developing(students discussion platform).
I want to when a student logs in , the system extracts and counts the number of only the newest posted articles( unread articles) posted by the other students. 
The problem is that it is only extracting those which match.
What could be the error</p>

<p><strong>TABLES:</strong></p>

<pre><code>-students(regno,name,faculty_code) PRIMARY KEY (regno)
</code></pre>

<p>-<code>articles(art_id,content,regno,faculty_code,date,time)</code></p>

<pre><code>-comments(com_id,art_id,co_content,regno,date,time)
</code></pre>

<p>i would like to 
<code>select articles.art_id and comments.art_id where articles.art_id is not equal comments.art_id and commens.regno='$</code> </p>