How to get Absolute value in SQL?

How do I get the absolute value from a field using SQL?

In the SQL below, SHAREAMOUT can be negative (sale), but since

the information I am gathering is a summary of transactions, the

average SHAREAMOUNT will get thrown off when it is negative.

select SUM(SHARES) as TOTALSHARES,

SUM(SHARES * SHAREAMOUNT) as TOTALVALUE,

AVG(SHAREAMOUNT) as AVGSHARE

from TransactionHistory

group by Symbol

EG:

Symbol ShareAmount

HMX 5

HMX -5

Averages to 0. What I want is 5--meaning of all the tranactions

the average per share value was 5.

Thanks

Matt