title: Error: The used SELECT statements have a different number of columns
body: <p>Why am I getting <code>ERROR 1222 (21000): The used SELECT statements have a different number of columns</code> from the following?</p>
  <pre><code>SELECT * FROM friends
  LEFT JOIN users AS u1 ON users.uid = friends.fid1
  LEFT JOIN users AS u2 ON users.uid = friends.fid2
  WHERE (friends.fid1 = 1) AND (friends.fid2 &gt; 1)
  UNION SELECT fid2 FROM friends
  WHERE (friends.fid2  = 1) AND (friends.fid1 &lt; 1)
  ORDER BY RAND()
  LIMIT 6;
  </code></pre>
  <p><code>users</code>:</p>
  <pre><code>+------------+---------------+------+-----+---------+----------------+
  | Field      | Type          | Null | Key | Default | Extra          |
  +------------+---------------+------+-----+---------+----------------+
  | uid        | int(11)       | NO   | PRI | NULL    | auto_increment |
  | first_name | varchar(50)   | NO   |     | NULL    |                |
  | last_name  | varchar(50)   | NO   |     | NULL    |                |
  | email      | varchar(128)  | NO   | UNI | NULL    |                |
  | mid        | varchar(40)   | NO   |     | NULL    |                |
  | active     | enum('N','Y') | NO   |     | NULL    |                |
  | password   | varchar(64)   | NO   |     | NULL    |                |
  | sex        | enum('M','F') | YES  |     | NULL    |                |
  | created    | datetime      | YES  |     | NULL    |                |
  | last_login | datetime      | YES  |     | NULL    |                |
  | pro        | enum('N','Y') | NO   |     | NULL    |                |
  +------------+---------------+------+-----+---------+----------------+
  </code></pre>
  <p><code>friends</code>:</p>
  <pre><code>+---------------+--------------------------------------+------+-----+---------+----------------+
  | Field         | Type                                 | Null | Key | Default | Extra          |
  +---------------+--------------------------------------+------+-----+---------+----------------+
  | friendship_id | int(11)                              | NO   | MUL | NULL    | auto_increment |
  | fid1          | int(11)                              | NO   | PRI | NULL    |                |
  | fid2          | int(11)                              | NO   | PRI | NULL    |                |
  | status        | enum('pending','accepted','ignored') | NO   |     | NULL    |                |
  +---------------+--------------------------------------+------+-----+---------+----------------+
  </code></pre>