There are at least four different types of SUM I have run across in SAS:
 In PROC PRINT
 The SUM statement, which causes PROC PRINT to display totals for the specified variable
 In the DATA step
 The sum statement, which adds a specified expression to an accumulator variable. The value of the accumulator variable is retained through each iteration of the data step. The sum statement treats missing values as 0.
 The + operator. This adds two numeric values, but if either value is missing the expression will evaluate to missing.
 The SUM function  this taks the form SUM(val1, val2, ...), returning the sum of the values, treating missing values as zero.
In PROC PRINT, SUM
variablename will give you the total value, for example:
PROC PRINT DATA=work.grocery;
var item;
sum price;
run;
Obs 
Item 
Price 
1 
Milk 
3 
2 
Tofu 
4 
3 
Bread 
5 


12 
On the other hand, in a DATA step, you have the sum statement
variable+expression. This initializes the value of
variable to 0 and increment by
expression in each iteration of the DATA step.
The SUM function and + operator have the same results, except in the case of a missing value.
MyVar = SUM(Var1,Var2);
MyVar = Var1 + Var2,
Will produce the same result, unless Var1 or Var2 is a missing value. If, for example, Var2 is missing, then the SUM function will return the value of VAR1, while the + operator will return the value missing. This is particularly confusing because the sum statement also uses the + symbol, but the sum statement treats missing values as zeros.