Warning null values are eliminated for aggregation or other set operations. What does that mean?

Cause:

Create Table TB
(
Id int,
Num int
)

Insert into TB select 1, 10
Insert into TB select 1, 20
Insert into TB select 2, 80
Insert into TB select 2, null

Select ID, sum (Num)
From TB
Group by ID

ID
----------------------
1 30
2 80

(The number of affected rows is 2)

Warning null values are eliminated for aggregation or other set operations.

Analysis: Aggregate functions cannot calculate null values, so they ignore
This prompt is only a warning, indicating that the null value is ignored.
The result is calculated as null is 0.

If you use
Select ID, sum (isnull (Num, 0 ))
From TB
Group by ID

Before the operation, isnull has converted the null value to 0,
So there is no problem with aggregate function operations.