This lets you apply criteria to determine which records to include in
the query. The WHERE option will filter the records before any
summing, counting, etc. take place, making it more efficient (and,
often, letting you create a query that returns correct answers).
Putting criteria on the Criteria line under a GROUP BY field can work,
but it's much less efficient since the query does all its summing,
counting, etc. and then discards any results which don't match the
criterion.
