mysql UNION query not working

Question

What am I doing wrong with this:

$sql = "SELECT * FROM content 
WHERE threadName LIKE '%$filter%' 
ORDER BY lastUpdated desc 
UNION SELECT * 
FROM content 
WHERE threadName NOT LIKE '%$filter%' 
ORDER BY lastUpdated desc";

The first statement before the UNION works well on its own, but this one above returns:

mysql_fetch_array() warning - supplied argument is not a valid MySQL result resource

Am I right in believing that UNION will not return duplicate entries, in which case the second SELECT statement doesn't need to have the NOT LIKE but will just return everything that wasn't listed in the first statement.

Answer 1

EDIT: This query should get you the rows matched by filter first, followed by those not matched:

SELECT *
FROM content
ORDER BY
  CASE WHEN threadName LIKE '%$filter%' THEN 0 ELSE 1 END,
  lastUpdated DESC

Note that you should never SELECT *, list the necessary columns instead.