How To Update A Column With The Sum Of Two Columns In A Table

I have a simple question that is more about performance on my code for updating a column with the sum of two columns:

Example

Table = dbo.adding

c1 c2 c3Sum
1 2
3 5
4 6

Now if I just simple write

Update dbo.adding
set c3SUM = c1 + c2

This works, but there is a null in c1 or c2 then I know I can write this:


if exists (select * from dbo.adding where c1 is NULL)

Update dbo.adding

set c1 = '0', c3SUM = '0' + c2 where c1 is NULL

if exists (select * from dbo.adding where c2 is NULL)

Update dbo.adding

set c2 = '0', equals = c1 + '0' where c2 is NULL

Update dbo.adding

set c3SUM = c1 + c2

This works as well, but I am not sure how perficient it is being that from my understanding the code will evaluate the table three times and update it based on the criteria. If the table consists of 100,000 rows then this might not be a good solution

Is there a more proficient way to do this... I personally try to as little coding as possible to help increase performance. I know I can make this very complex and declare variables and put everything in a loop, but I personally think that would be less profient..