Control Structures 

To execute different program parts in connection with certain conditions or to combine recurring statement sequences as loops, you can use standard keywords like IF, CASE, DO, and WHILE in the flow control of a program.

For controlling the flow of an ABAP program, remember that the structure of the program  is made up of processing blocks  that cannot be nested. This section describes how to control the flow of a program within a processing block. The keywords covered here do not allow you to branch outside the processing block in which you use them. You can regard this as internal control of an ABAP program, as opposed to the external control provided by events in the ABAP runtime environment.

To control the internal flow of a processing block, you can use control structures to divide it up into smaller statement blocks according to the principles of structured programming. Each statement block takes care of one sub-task.

Unlike processing blocks, control structures can be nested. In the following section, you will learn more about branching conditionally and about loops