I have some code like this:

```
Update table_name
set
[column] = case when d.data is null, then null else d.columnname end.

from...
etc

```

My question is, how do I set up a function where the 'else d.columnname' is to sum up several columns from joins. 

Would it be something like:

```
 ...then null else sum(d.column1 + rf.column2 + rwf.column3) as tempcolumn end,

```

or

```
...then null else (d.column1 + rf.column2 + rwf.column3) end,

```

What is the correct way to do a column sum in this set situation?