Querying(Chapter 4 of MongoDB The Definitive Guild)
时间：2010-10-01 来源：Ray Z
Sometimes, you do not need all of the key/value pairs in a document returned. If this is the case, you can pass a second argument to find (or findOne) specifying the keys you want. This reduces both the amount of data sent over the wire and the time and memory used to decode documents on the client side.
> db.users.find({}, {"username" : 1, "email" : 1})
{
"_id" : ObjectId("4ba0f0dfd22aa494fd523620"),
"username" : "joe",
"email" : "[email protected]"
}
{
"_id" : ObjectId("4ba0f0dfd22aa494fd523620"),
"username" : "joe",
"email" : "[email protected]"
}
"$lt", "$lte", "$gt", and "$gte" are all comparison operators, corresponding to <, <=, >, and >=, respectively.
There are two ways to do an OR query in MongoDB. "$in" can be used to query for a variety of values for a single key. "$or" is more general; it can be used to query for any of the given values across multiple keys.
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