Franko | Arduino - Self balancing robot

As far as I can see your nested listener only loads the score for a single player, so it shouldn't be looping over the child nodes in onDataChange. By doing that, your it becomes an individual property, and trying to read a from a single property value is leading to the message you get.

It more likely should look something like this:

The database doesn't attempt to match rows from the two tables - it performs a Cartesian product (i.e., matching every row from the first table with every row of the second table), and it's up to you to provide a condition telling the database how to join these rows.

In the first example, you don't have a condition, so you're getting 2*2 = 4 rows. This is called a cross-join. In the second example, the condition of 1=1 always evaluates as true, so although it doesn't like it, it's also a cross-join, and produces the same results as the first query.

Assuming you meant to match the rows with the same IDs, you need to provide this logic as a condition:

There are several errors in your code, you try to return multiple times and you can use filter instead of your second map:

Once you have the $name variable, you can simply query table 1 with a select statement to find if information for a specific student exists in table 1 (SELECT * from table1 WHERE NAME=$name). If this query returns any results, perform the insertion into table 2 which you mention above. Otherwise, do nothing.