Firebase querying data

In your case you would have to setup a Query like this:

DatabaseReference reference = FirebaseDatabase.getInstance().getReference();

Query query = reference.child("issue").orderByChild("id").equalTo(0);
query.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        if (dataSnapshot.exists()) {
            // dataSnapshot is the "issue" node with all children with id 0
            for (DataSnapshot issue : dataSnapshot.getChildren()) {
                // do something with the individual "issues"
            }
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});

This code works for me

Query query = mFirebaseDatabase.child("issue").orderByChild("id").equalTo(0)
    query.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(final DataSnapshot dataSnapshot) {
                    for (DataSnapshot data : dataSnapshot.getChildren()) {
                     //If email exists then toast shows else store the data on new key
                        if (!data.getValue(User.class).getEmail().equals(email)) {
                            mFirebaseDatabase.child(mFirebaseDatabase.push().getKey()).setValue(new User(name, email));
                        } else {
                            Toast.makeText(ChatListActivity.this, "E-mail already exists.", Toast.LENGTH_SHORT).show();
                        }
                    }
                }
    
                @Override
                public void onCancelled(final DatabaseError databaseError) {
                }
            });

@Linxy's answer is correct but since you'll be reading a list of items from the database, it's better to use a child event listener instead of the value event listener.

DatabaseReference reference = FirebaseDatabase.getInstance().getReference();

Query query = reference.child("issue").orderByChild("id").equalTo(0);
query.addChildEventListener(new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String s) {
        //Do something with the individual node here`enter code here`
    }

    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String s) {

    }

    @Override
    public void onChildRemoved(DataSnapshot dataSnapshot) {

    }

    @Override
    public void onChildMoved(DataSnapshot dataSnapshot, String s) {

    }

    @Override
    public void onCancelled(FirebaseError firebaseError) {

    }


});