Use of Nodes:
When you use a logical database, you can control the selection screen and the depth to which the logical database reads data. With NODES statement, you specify the nodes of the logical database that you want to use in the executable program. The statement declares table work areas for these nodes in the program. These are data objects with the same name and structure as the corresponding node. The selection screen of the logical database only contains input fields for the nodes that you have declared using NODES.
NODES CAN ONLY BE REFERRED WITH A PARTICULAR LOGICAL DATABASE:
When you create an executable program, you can specify the name of a logical database in its program attributes. When you run an executable program that has a logical database linked to it, the two programs function like a single executable program whose processing blocks are called by the runtime environment in a particular order.
Executable programs that do not use a logical database, but define their own selection screen and read data using Open SQL statements (SELECT) often require only one single processing block – the standard event block START-OF-SELECTION. In this case (and only in this case), an executable program behaves like a classic sequentially-processed program. However, as soon as you process a selection screen in the program (for example, filling input fields before the call or checking user input after the call), you have to include further processing blocks (INITIALIZATION, AT SELECTION-SCREEN). If you use a logical database, the program is fully event-driven. The most important event used with logical databases is GET. At this time, the logical database passes an imported record to the appropriate processing block in the executable program.
For details have a look at below links.
http://help.sap.com/saphelp_nw2004s/helpdata/en/9f/db99b435c111d1829f0000e829fbfe/content.htm
http://help.sap.com/saphelp_nw2004s/helpdata/en/60/183d78163011d2953c0000e8353423/content.htm
I hope it helps.
Best Regards,
Vibha
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