Difference between "+" operator, SUM function, SUM statement

[RAVI2000]

There are 2 variables A and B, and I create variable C using + operator, variable D using sum
function and E using sum statement. What will be result?

I know the difference between "+" and SUM function.

"+": If there are any missing values it returns missing result.

SUM function: It doesn't consider missing values and averages the remaining data.

Some information about SUM statement is, it has default functionality as RETAIN statement. 

I want to know how it is working. Can any one give me a scenario?

Thankyou!

[Kurt_Bremser]

Here a typical use of the SUM statement to create a running count and sum per group:

data want;
set have;
by id;
if first.id
then do;
  count = 1;
  sum_amount = amount;
end;
else do;
  count + 1;
  sum_amount + amount;
end;
run;

As the SUM statement implies a retain, no separate RETAIN is needed.

[RAVI2000]

How can I explain the SUM statememt as?
It gives cumulative sum by retaining ?

[PaigeMiller · Accepted solution]

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@RAVI2000 wrote:
How can I explain the SUM statememt as?
It gives cumulative sum by retaining ?

---

It gives a cumulative sum by retaining. In the code below, it adds the value of X to the previous value of CUMULATIVE_SUM

data a;
    input x;
    cumulative_sum+x;
cards;
5
1
-7
12.4
;

A slight correction

SUM function: It doesn't consider missing values and averages the remaining data.

SUM function: It doesn't consider missing values and sums the remaining data.

[RAVI2000]

Thank you @Kurt , for the clear explanation.