How do I do math with a UNION ALL

Hi Everyone.

I am trying to do a SUM of the "impressions" and "usage" columns from 2
different tables using a UNION ALL.

I have tested that the individual SELECT queries work. I am unsure of the
layout for the UNION ALL. Would you help me tweak this query?

I also wanted to ask is "usage" a reserved word in mySQL? Is this the
reason the query fails?

Ron

===

SELECT SUM( impressions ) AS "impressions", SUM ( usage ) AS "usage" FROM (

(

SELECT IF ( SUM( `web_advertisements_our_clients_usage`.`impressions` ) ,
SUM( `web_advertisements_our_clients_usage`.`impressions` ) , 0 ) AS
"impressions", IF ( SUM( `web_advertisements_our_clients_usage`.`usage` ) ,
SUM( `web_advertisements_our_clients_usage`.`usage` ) , 0 ) AS "usage" FROM
`web_advertisements_our_clients_usage` WHERE
`web_advertisements_our_clients_reference` = 1 AND `month` = 4 AND `year` =
2011 LIMIT 1

) UNION ALL (

SELECT `impressions`, `usage` FROM `web_advertisements_our_clients` WHERE
`reference` = 1 LIMIT 1

)

) AS monthly_stats_total;


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