title: What does the KEY keyword mean?
body: <p>In this MySQL table definition:</p>
  
  <pre><code>CREATE TABLE groups (
    ug_main_grp_id smallint NOT NULL default '0',
    ug_uid smallint  default NULL,
    ug_grp_id smallint  default NULL,
    KEY (ug_main_grp_id)
  );
  </code></pre>
  
  <p>What does the <code>KEY</code> keyword mean? It's not a primary key, it's not a foreign key, so is it just an index? If so, what is so special about this type of index created with <code>KEY</code>?</p>