I'm trying to get the sum of a column from my database. When I try this code I get the error `Undefined index: sum(col_1)`. How can I do this? I'm a real beginner so keep it a bit simple.

```
$sql = "SELECT * FROM table_1";
$run = mysqli_query($conn, $sql);
while($rows = mysqli_fetch_assoc($run)){
    $col_ans[1] = $rows['sum(col_1)'];
        echo $col_ans[1];
    }

```