query not calculting using declared variables

[bravo14]

Hi

 

I have a mysql query that generates a league table based on wins, losses and draws etc, but also points difference between how many they score and how many are scored against them.

 

The query works except for the points difference part, the offending part of the query is shown below

 

 

@F:=SUM(IF(tbl_clubs.club_id = tbl_fixtures.away,tbl_fixtures.awayscore,0)) +
SUM(IF(tbl_clubs.club_id = tbl_fixtures.home,tbl_fixtures.homescore,0)) as `F`,

@A:=SUM(IF(tbl_clubs.club_id = tbl_fixtures.home,tbl_fixtures.awayscore,0)) +
SUM(IF(tbl_clubs.club_id = tbl_fixtures.away,tbl_fixtures.homescore,0)) as `A`,

SUM(@F - @A) as `PtsDiff`,

 

Using the code above I am getting NULL as a result even though there should be a figure calculated.