Get only the final value of the bank

I am using php with pdo to query a database in postgress .

Is there any way that when I do query , show the result without having to insert the table name, since the select is unique?

select distinct a as origem 
 from b union select distinct  as dest form b

php

for ($x = 0; $x < $stmt->rowCount(); $x++) {
    $n[] = $stmt->fetch(PDO::FETCH_ASSOC);
    }

This way if you put the table name, I know that if you do $row['origem'] it works but there may be dest too.