Question: I am new to SAS and would like to know what are the difference Difference between "*IF-THEN*" and "*IF-THEN-DO*" statements in SAS?

Answer: 
SAS evaluates the expression in an IF-THEN statement to produce a result that is either non-zero, zero, or missing. A non-zero and nonmissing result causes the expression to be true; a result of zero or missing causes the expression to be false.

If the conditions that are specified in the IF clause are met, the IF-THEN statement executes a SAS statement for observations that are read from a SAS data set, for records in an external file, or for computed values. An optional ELSE statement gives an alternative action if the THEN clause is not executed. The ELSE statement, if used, must immediately follow the IF-THEN statement.

Using IF-THEN statements without the ELSE statement causes SAS to evaluate all IF-THEN statements. Using IF-THEN statements with the ELSE statement causes SAS to execute IF-THEN statements until it encounters the first true statement. Subsequent IF-THEN statements are not evaluated. (Source: support.sas.com)

The DO statement is the simplest form of DO group processing. The statements between the DO and END statements are called a DO group. You can nest DO statements within DO groups.

A simple DO statement is often used within IF-THEN/ELSE statements to designate a group of statements to be executed depending on whether the IF condition is true or false. (Source: support.sas.com)

Regards,
Vasilij
Simplified you can say, if then is for one statement, if then do for a block of statements. If you use if without then in Datastep, it prevents output for the specific set.

Example:

```
data x;

set y;
if a = 1 then /*one statment is following*/
b=2;

if a = 1 then do; /* a block of statements is follwing till end statement, similar to brackets in other programming languages*/
b=2;
c=3;
end;

if a = 1; /*only when a = 1 data will be written to x*/

run;

```