db.customers.find({first_name : "Sharon"});
db.customers.find({$or : [{first_name : "Sharon"}, {first_name : "Troy"}]});
db.customers.find({gender : "male"});
db.customers.find({age : {$lt : 40}});
// there are also $gt, $lte and $gte
db.customers.find({"address.city" : "Boston"});
db.customers.find({memberships : "mem1"});