SQL Alias question

[LSW]

In my query I have a mathematic function under an alias:

 (a amount+ b amount) AS "difference" 

 

Now however I need to sum the difference, but apparently you cannot sum information using an alias:

 SUM('difference') AS "total_difference" 

 

I get a ORA-01722: invalid number error.

 

Any ideas?

[LSW]

Nope, I guess you can't. Now if I could figure out how to get it in my Crystal Report without mucking up the other sums...