To understand database tables in SAP special database better, we have to first understand SAP's architecture. Here is a small glimpse into SAP's 3-tier architecture. This is the UI layer. When you log into SAP using the SAP GUI, this GUI can be called the UI layer. You can also log in using the browser special database based Web UI. This is the Application Server Layer. This is the core of SAP. This comprises of the runtime environment of SAP. At a high level there is a dispatcher that looks at the incoming request from the front end and allocates a work process ( WP ) to take care of the request. There are a lot of other processes involved in SAP's runtime environment and they are beyond the special database scope of this ABAP tutorial. For now it suffices to know that an ABAP program's runtime environment is in the Application Server's memory.
This is where the database resides. SAP has been built in a database agnostic way. There are a list of databases supported by SAP and as long as you use one of those databases in tier-III SAP will work. Now, we all know that each of these databases have their own special database versions/flavors of SQL. So when SAP's Application server interacts with the database layer, it talks via the "Database Interface" or simply DBIF. The task of DBIF is to ensure that it converts SAP's Open SQL into native SQL understandable by the corresponding special database database.
Without going much further into SAP's architecture, let's start by vcreating a database table in special database SAP. Let's try to create a database table "Customer" as follows. SAP's upgrade ensures that those programs special database are not touched. It is however the responsibility of the customer to ensure that all those custom programs work with special database the new version of SAP - This is essentially what an upgrade project involves anyway.