Need help creating Oracle SQL Expression in IDT

I am admittedly new to Webi, and need some assistance getting my SQL to work in the SQL Expression Editor of the Business Layer.  The SQL that I am trying to get to work looks like this:

select

"Route ID",round((sum("Standees") * sum("Passengers")/sum("Passengers Checked")),0) AMTRAK_RIDERS

from

"RPT_TRIPS"

Where

trunc("Trip Date")between'01-JUL-2012'and'31-JUL-2012'

and

to_char("Trip Date",'D')notin('1','7')

group

by"Route ID"

order

by"Route ID"    I would like to know how to reference it using @select and the measures from the Business Layer. I am struggling with that syntax.