{
  "users": {
    "user1": {
      "name": "John",
      "age": 25
    },
    "user2": {
      "name": "Jane",
      "age": 30
    },
    "user3": {
      "name": "Bob",
      "age": 25
    },
    "user4": {
      "name": "Alice",
      "age": 30
    }
  }
}

var ref = firebase.database().ref("users");
var query = ref.orderByChild("age").equalTo(25);

query.once("value", function(snapshot) {
  var count = snapshot.numChildren();
  console.log("计数：" + count);
});