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Firebase Realtime database Array query issue [duplicate]

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Firebase query if child of child contains a value
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I need a firebase query to filter the list based on the value of an array.
if any of the index of GID(Array) contains the given key. e.g my key is YsGMyfLSGRNHxDWQmhpuPRqtxlq1 and one node's GID have that on 0th index and other have that on 1st index. So these two lists need to be returned.
Currently, I can only get the one at 0th index using the code
//userID = YsGMyfLSGRNHxDWQmhpuPRqtxlq1
firebaseDatabase.child("Groups").queryOrdered(byChild: "GID/0").queryEqual(toValue:userID)
When I try to combine the query I am getting errors.

I don't know about your database structure, But I can explain that There is a limitation in Firebase Realtime database that you can only order by 1 child.
So now if we require to order by 2 Childs we can combine 2 nodes and make it 1 node and can apply order by query on it. For example
If we have username & email fields we can make a new field username_email and can apply order by on it.
Like
user: {
username: "john",
email: "john#g.com"
username_email = "john_john#g.com"
}
Now we can write
firebaseDatabase.child("user").queryOrdered(byChild: "username_email").queryEqual(toValue: "john_john#g.com");

There is no way you can filter your groups based on a value that exist within an array. If you want to query your database to get all groups a particular user is apart of, then you should consider augmenting your data structure to allow a reverse lookup. This means that you should add under each user object the groups in which that user is present.
This means that you'll need to duplicate some data, but this is not a problem when it comes to Firebase. This is a quite common practice, which is named denormalization and for that, I recommend you see this video, Denormalization is normal with the Firebase Database.
When you are duplicating data, there is one thing that need to keep in mind. In the same way you are adding data, you need to maintain it. With other words, if you want to update/detele an item, you need to do it in every place that it exists.
However, what you need is actually allowed in Cloud Firestore. Its array-contains operator allow you to filter documents that have a certain value in an array. For more on this topic, please see the following post:
Better Arrays in Cloud Firestore.

Not know exception with firebase filters

This code:
var query: Query = store.collection(COLLECTION_USERS)
query = query.whereGreaterThanOrEqualTo("age", filter.startAge).whereLessThanOrEqualTo("age", filter.endAge)
query = query.whereGreaterThanOrEqualTo("start_date", filter.startDate).whereLessThanOrEqualTo("end_date", filter.endDate)
query = query.whereEqualTo("sex", filter.sex)
return query.get()
throw exception: java.lang.IllegalArgumentException: All where filters other than whereEqualTo() must be on the same field. But you have filters on 'age' and 'start_date' how to fix?

Official documentation of Firestore says that range filters on different fields are forbidden. So Firestore allows to chain multiple where() methods to create more specific queries but only on the same field.
To achieve what you want, you need to query your database twice, once for each fiter because you cannot use both methods in the same query.
Another way to make it happen would be to store a special flag that might fit the query, although in real world applications it will almost impossible to store every single way a user might query the data.

Firebase Database - How to perform a prefix wildcard query

I would like to perform wildcard queries on a Firebase database,
and do not know whether the libraries support this. - (my guess it does not.)
I want an optimal solution where the network traffic will be as little as possible, assume the dataset is millions of records.
Dataset (using prefix wildcard):
user_id:
id_001123:
name: "java"
id_002124:
name: "objective-c"
id_003125:
name: "swift"
How would the code look like to retrieve the record id_002124 name field using a wildcard, you only have a portion of the id. eg. "%124".
The expected result would be id_002124, and the name being "objective-c"
Any mobile language example code would great.
(Objective-C/Swift/Java)

Firebase Database queries can order/filter data based on the object's keys (e.g. id_001123), their values (doesn't apply in your case), or the value of a child property (e.g. the fact that name = "java").
For each of these Firebase can match items that have a specific value you specify, items starting at a value you specify, or items ending at a value you specify.
So you can create a query matching the item named objective-c with:
ref.child("user_id").orderByChild("name").equalTo("objective-c")
Or you can create a query matching id_001124 and id_001125 with:
ref.child("user_id").orderByKey().startAt("id_001125")
Or simply getting all items starting with id_0011:
ref.child("user_id").orderByKey().startAt("id_0011")
Or you can create a query matching id_001123 and id_001124 with:
ref.child("user_id").orderByKey().endAt("id_001124")
Firebase Database cannot filter based on the end of a value. So there is no way in your current data structure to get all items whose key ends in a 4.
Please read more about Firebase Database queries here: https://firebase.google.com/docs/database/android/lists-of-data#sorting_and_filtering_data
And see some of the many previous questions about searching for FirebaseL
How to perform sql "LIKE" operation on firebase?
Firebase query - Find item with child that contains string
Firebase "like" search on string

Firebase query implementation

If we query data from Firebase on a key for a child node, does it downloads the whole child node and filter data in the application or it downloads the query specific data? i.e. the filtered data
String myUserId = getUid();
Query myTopPostsQuery = databaseReference.child("user-posts").child(myUserId).orderByChild("starCount");
myTopPostsQuery.addChildEventListener(new ChildEventListener() {});
myTopPostsQuery will sort data according to starCount and I will receive data in the addChildEventListener() I want to know that whether this data is being filtered inside my app after receiving or my app downloads only the filtered data from the Firebase.

If you use a filter in your query you will download the query specific data without any operation executed on the client side.
Keep in mind that:
You can only use one order-by method at a time. Calling an order-by method multiple times in the same query throws an error.
You can combine multiple limit or range functions. For example, you can combine the startAt() and endAt() methods to limit the results to a specified range of values.
For any other info take a read here

You get the data already filtered. When you use a query, let say, limitToLast(10) you get only those 10 elements. This is happening also in your case with orderByChild("starCount"). In your SnanpShot will find only those filtered elements. Please visit official doc for more details.
Hope it helps.

Based on the code you pasted here, your query will just retrieve all the posts for the database path user-posts/<id> with an ordered manner which means that there is not filter. But still you will get back all the available posts under the path you are querying. It can be a "semi-filter" as it will find only the posts which include starCount field
The best thing is to filter during your query in order to retrieve back exactly what is needed and not everything as you are doing right now, imagine that this list of posts can be really big so you will have a big issue with performance later.
Read the following section here about sorting & filtering.
https://firebase.google.com/docs/database/admin/retrieve-data#orderbychild
Apart from that consider to add an index in userId field for speeding up your query.

Can Firebase return only some of the attributes? [duplicate]

Is there any way to select specific properties from firebase Realtime Database? I know there is a way to retrieve selected properties from firestore but how can get via Realtime database using Node.js
I want only Notes from everyone nothing else.
Suppose i just want to select Notes from Allergy here is my sample code which i tried but not successes...
admin.database().ref(`vitals/Allergy`).select('Notes').then(result => {//here is my result.....})
But it shows me that select is not a function.......

Realtime Database doesn't support "projections" like this (neither does Cloud Firestore). If you are going to query across multiple child nodes, you are going to get each entire child node that matches the query. Even if you want just one property of each child, you can't avoid the cost of downloading the entire child.
If your app is very sensitive to performance on these types of queries, consider duplicating the data such that there is another branch of your database that contains only the "Notes" property, and query that branch alone. This duplication is common in NoSQL type databases, and is call the "fan out" technique.

If you want to save download band then i can't help you further, otherelse:
let ArrayOfAllDownloadedNotes = [];
firebase.database().ref('yourRootFolders/vitals/Allergy').once((snapshot)=>{
ArrayOfAllDownloadedNotes = [...ArrayOfAllDownloadedNotes, snapshot.val().Notes];
});
//Reapeat the firebase formula for every child you want retrieve the Notes from
//You may also use forEach((item)=>{}) function for each folder you want to retrieve the //notes from if you want
console.log(My Array of Notes:',ArrayOfAllDownloadedNotes );