Develop Reference

Get specific child with same parent as another child Firebase

I am making an Android app in Android Studio and I want to get specific data from the Firebase. I am trying to search through the Firebase Realtime Database to get all of the Users that have a usercode of 1234 and then getting their name. This is how I have it set up in Firebase:
Firebase Setup image link
Since I am searching for the data where the usercode = 1234 it will return (in the datasnapshot) all of the information about Joe and Lily (since they were the two users that had a usercode of 1234), but it shouldn't return information about Kevin. Here is how I did that:
Query getUsers = ref.orderByChild("usercode").equalTo(1234);
getUsers.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
if(dataSnapshot.exists()){
//User with usercode = 1234 exists
} else {
//User with that Usercode doesn't exist
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
}
The datasnapshot, if converted to a string would have all of Lily and Joe's information I believe.
What I want to do is get the first user's name (Which would be Joe in this case) from that datasnaphot. My code will then use it through a few tests before going to the second user's name(Which would be Lily) and repeating the tests. Also, if there were 100 Users then I would need to repeat it for every one of those 100 that had a user code of 1234, so I probably need something repeatable.
So somehow I need to find the first value of the child's name from all of the Users that have a usercode of 1234 and then repeat until I get all of the names and run the test on all of them.
I am new to Stackoverflow, so please tell me if I am missing any information that would be helpful in answering the question.
---- EDIT ----
I have now figured out how to do this with Firebase Firestore. It has simpler queries and allows you to do more with the results. If anyone else has the same issue that I stated above, Firebase Firestore is something you might want to try out.

When using the following line of code:
Query getUsers = ref.orderByChild("usercode").equalTo("1234");
You are searching through the Users node, all users that have the usercode property set to "1234". This query will always return no results because you are searching for 1234 as a String and not as a number, as it is stored in the database. The correct query should be:
Query getUsers = ref.orderByChild("usercode").equalTo(1234); //No double quotes

Firebase Query startAt()

I am using a query and having some trouble. I want retrieve value from 5th to 10th. I try to use startAt () the query just doesn't get anything.
public void read(){
Query query = FirebaseDatabase.getInstance().getReference("values").orderByChild("timestamp").startAt(5).endAt(10);
query.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
for(DataSnapshot data: dataSnapshot.getChildren()){
Value upload = data.getValue(Value.class);
mUploads.add(upload);
adapter.notifyDataSetChanged();
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
Toast.makeText(MainActivity.this, "Loading Quote failed", Toast.LENGTH_SHORT).show();
}
});
}

Tl;DR: Firebase database queries are cursor based, and not offset based.
When you call startAt() you have to pass in the value of the timestamp you want to start returning results at. You can't pass in offsets, as you are trying to do now.
You current code:
FirebaseDatabase.getInstance()
.getReference("values")
.orderByChild("timestamp")
.startAt(5)
.endAt(10);
Tells the database to:
Find the node called values under the root.
Order each child node by its timestamp value.
Then find a child node with value 5 and start returning results from there.
Stop returning results once it finds a child node with value 10.
As you can probably see, this won't give you any results, since none of your child nodes have a timestamp value between 5 and 10.
As said: Firebase queries are cursor based, meaning they work on knowing something about the node to start at. While many databases paginate based on offsets, Firebase doesn't and you'll need to know the timestamp value of the node to start at.
For example, if you want to start at the node you have opened in your screenshot, and then return the next 5 results, you'd do:
FirebaseDatabase.getInstance()
.getReference("values")
.orderByChild("timestamp")
.startAt(-1590413110563)
.limitToFirst(5);

How to Optimize Firebase Data Read

I am trying to understand how firebase data read is count.Is there any way to use hashmap so that whole data read is count as a single read. I have below code currently to fetch data from firebase where I am saving each child value in separate string variable to use in another purpose. Not understanding is it counting as a single read or multiple. is there any better way to do this?
reference.orderByKey().equalTo(bidPosition).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot datas : dataSnapshot.getChildren()) {
if(dataSnapshot.hasChildren() && !bidPosition.isEmpty()){
String eta = datas.child("eta").getValue(String.class);
String riderid = datas.child("uid").getValue(String.class);
String drivierid = datas.child("did").getValue(String.class);
String drivier_ratings = datas.child("dRat").getValue(String.class);
String r_Fare = datas.child("tAmt").getValue(String.class);
}
}

Firebase Realtime Database isn't billed in terms of number of reads. You might be confusing it with Firestore, which is billed, in part, based on the number of documents read.
For Realtime Database, the pricing page makes it clear. You are charged for $5/GB stored (regardless of how it's read), and $1/GB downloaded. How you download/read the data doesn't matter - it's the total volume that counts. So there is really no way to significantly optimize a read or query operation that fetches some data.

Find max value from last 50 nodes from Firebase database

I want to find max value from only last 50 nodes. I used to orderByChild and limitToLast methods but it finds max from all nodes, not only from last 50.
Here my db:
And my code:
database = FirebaseDatabase.getInstance();
reference = database.getReference("sensor");
Query query = reference.orderByChild("hum").limitToLast(50);
query.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot myDataSnapshot : dataSnapshot.getChildren())
{
humTemp = myDataSnapshot.child("hum").getValue().toString();
humMax.setText(humTemp);
}
}
public void onCancelled(DatabaseError firebaseError) {
}
});
So, as you see i try to find max value of hum variable from last 50 nodes.
P.S. New nodes are constantly appearing in the database, so the maximum value should change for every 50 values in realtime

If you want the maximum of the 50 most recent nodes, you're trying to order by two properties:
humidity
the time of the node (or its key)
Firebase Database queries can only order/filter on a single property.
In many cases it is possible to combine the values you want to filter on into a single (synthetic) property. For an example of this and other approaches, see my answer here: http://stackoverflow.com/questions/26700924/query-based-on-multiple-where-clauses-in-firebase. But I don't think that's possible for you.
The best I can think of is to just listen for the last 50 nodes, and then re-ordering them on humidity in the client.

Android: Firebase query single word among values

I want to use Firebase real-time database in my app project. I want to check if the word which is filled by user exists in database.
Here is my database ss:
Here is my code:
private Button button;
private FirebaseDatabase database;
private DatabaseReference mRef;
private TextView tvResult;
private EditText et1;
et1=(EditText) findViewById(R.id.et1);
button=(Button) findViewById(R.id.btn);
tvResult=(TextView) findViewById(R.id.tvResult);
database=FirebaseDatabase.getInstance();
mRef=database.getReference();
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
city=et1.getText().toString();
Query query = mRef.child("app").orderByChild("cities").equalTo(city);
query.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
if (dataSnapshot.exists()) {
// dataSnapshot is the "issue" node with all children with id 0
String get= dataSnapshot.getValue().toString();
tvResult.setText(get);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {}
});
}
});
As I said, user fills edittext and after clicking a button, I want to show written city in textview if there is in database.

What you have in your cities is a set of values: a collection of values, where each value is unique, and the order doesn't matter. So it's a single property with multiple values. And while NoSQL databases are all about denormalizing the data, this is actually one of the cases where I'd recommend that you normalize it a bit.
In the Realtime Database you'll typically model a mathematical set as this JSON:
cities: {
london: true,
paris: true,
milan: true,
ny: true,
istanbul: true
}
This data structure has the advantage that:
Each city is by definition unique, since keys must be unique in their location. I.e. there's no way to have london in here twice.
Each city is now stored as a separate property, which means you can query for it separately.
For example, if you have a list of say trips where each trip can have a cities node, and you model it as I've shown, you could query for all cities that include New York with:
firebase.database().ref("cities").orderByChild("cities/ny").equalTo(true)
This will technically work, but will perform pretty badly, because of the way Firebase Realtime Database indexes work. For more on that, and an explanation of how to model this to allow the use-case, have a look at my answer here: Firebase query if child of child contains a value.
I definitely recommend that you check Cloud Firestore out. This use-case is precisely one where Firestore recently released some big improvements. In Firestore you can now store your data set in an array:
["london", "paris", "milan", "ny", "istanbul"]
So say that again you have a collection of trips, and for each trip you have a field cities whose value is the array above. You can now query for trips that include New York with:
firebase.firestore().collection("trips").where("cities", "array-contains", "ny")
And unlike with the Realtime Database, on Firestore this query will scale very well. For more on this see the blog post Better Arrays in Cloud Firestore!.

Unfortunately, there is no query in Firebase real-time database that looks something like this:
mRef.child("app").orderByChild("cities").contains(city)
But for small datasets, there is a workaround that can help you solve this, which is by querying the database to get all the value for your cities property and use contains() method like this:
String str1 = dataSnapshot.child("cities").getValue(String.class);
String str2 = "london";
boolean b = str1.toLowerCase().contains(str2.toLowerCase());
If you'll try to print the value of b, you'll see that is true. But the above examples works well enough only for very small datasets, it doesn't work for large datasets and this is because downloading an entire object to search for fields client-side isn't practical. In this case, I recommend use a third-party search service like Algolia.