mysql UNION query not working - mysql

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.

Answer 2

While using UNION we must use column names instead of '*'.
Here, I m going to create .csv file at specific location on the system with emailid validation expression by using mysql query as bellow.
select 'col1','col2','col3','col4' from tableName
UNION
SELECT col1,col2,col3,col4 FROM tableName WHERE col4 NOT REGEXP '^[a-zA-Z0-9][a-zA-Z0-9.-]*[a-zA-Z0-9.-]#[a-zA-Z0-9][a-zA-Z0-9._-]*[a-zA-Z0-9]\.[a-zA-Z]{2,4}$'
INTO OUTFILE '/home/sachin/mysqloutput/data.csv'
FIELDS TERMINATED BY ','
ENCLOSED BY ''''
LINES TERMINATED BY '\n';