Remember that SUM may return NULL, so SUM(num1)+SUM(num2) may lead to NULL:

select

sum(num1) AS S1,

sum(num2) AS S2,

sum(num1) + sum(num2) AS S3,

sum(num1 + num2) AS S4

from

(

select cast('1' as int) as num1, cast(NULL as int) as num2

union all

select '1' as num1, NULL as num2

union all

select '1' as num1, NULL as num2

union all

select NULL as num1, NULL as num2

) as t

The above select return 3,NULL,NULL,NULL