I have a two table table_a and table_b , table_a has a field a and table_b has a field b. 
a  b 
1  3
2  4
3  5
4  6
I have been written below sql & php code
$query  =  "select * from table_a FULL OUTER JOIN table_b on table_a.a = table_b.b";
$result = $conn->query($query);
while($row = $result->fetch_assoc()){   //this is 38 number line
        echo "<pre>";
            print_r($row);
        echo "</pre>";
}
It's giving me below error
Fatal error: Call to a member function fetch_assoc() on a non-object in C:\xampp\htdocs\sql\index.php on line 38
I am not finding any error here. If I use inner join here it's showing me result correctly.
 
    