SQL Syntax Help - Update

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 开发者_高级运维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?

---

You can simply do:

update MyTable
set column =
    case 
        when d.data is not null 
        then d.column1 + rf.column2 + rwf.column3 
    end
from ...

CASE will return NULL by default when there is no match.

---

Something like this should work:

UPDATE table_name
   SET [column] = CASE WHEN d.data IS NULL
                          THEN null 
                          ELSE (d.column1 + rf.column2 + rwf.column3)
                  END
  FROM table_name
      INNER JOIN other_table1 d ON ...
      INNER JOIN other_table2 rf ON ...
      INNER JOIN other_table3 rwf ON ...

Of course, in the query above you have to put in the correct relations between the tables in the INNER JOIN - ON clauses