When using a UNION in your Select Statement, you must ensure you use a WHERE statement to return the correct records.

I am using this example.
Select Ordered from Tbl1 where ID=1
The above statement will return all records with the ID of 1.
Now, let's say we have our table connected to another table to ensure the returned ordered numbers are correct.
We could do this.
SELECT Ordered FROM Tbl1 UNION SELECT Ordered FROM Tbl2 where ID = 1
Now, this will return the records for ID=1. However, this will also return records for 2, 3, 4, 5, etc.
This code lacks one thing: it does not look at where Tbl1 needs to get its records from.
To retrieve the records for each Query, we need to ensure we have a WHERE Statement at every corner.
SELECT Ordered FROM Tbl1 where ID = 1 UNION SELECT Ordered FROM Tbl2 where ID = 1
The above code will now only retrieve records for the ID that is the same across both tables.