I need to populate a currency value column with a value which is calculated from 2 other columns.

So:

```
original_value    amount         new_value
12                  2             NULL
10                  1             NULL

```

This would become:

```
original_value    amount         new_value
12                  2             24
10                  1             10

```

I only want to update the NULL columns.

This needs to work for SQL Server and MYSQL!