title: Mysql query function "select"
body: <p>The query <code>SELECT * FROM table WHERE ...</code> scrolls through the list and returns the corresponding elements? or use other ways?
  If I have a table of 10000 lines what makes the query?
  Here is an example used for a login:
   </p>
  
  <pre><code>$result = mysql_query ("SELECT * FROM users WHERE username = '$ username' AND password = '$ password'", $con);
  
  $row = mysql_fetch_array ($ result);
  </code></pre>
  
  <p>My question is, if I have a table with many rows does not become heavy?</p>