Query within Query (SubQuery)

I was searching through some Borland C++ querys to an ACCESS database and I could not find some of the tables being referred to in the code. I was about to go ask my boss, when it hit me that perhaps these were not table but internal queries in ACCESS. Yes, that is what they were.

This is about 12 pages of information on subquerys: http://www.aspfree.com/c/a/MS-SQL-Server/Subqueries-and-Query-Expressions/

Example:

SELECT city
FROM offices
WHERE target >
(
SELECT SUM(quota)
FROM salesreps
WHERE rep_office = office
)