Generate Database diagram within SAP

Generating the database diagram completely as generated in SQL server or other DBMS is not possible over here. But there are couples of ways that can be of small help in your case. 

Firstly, you can refer to Object Model guide present in the SAP market place. This guide will explain you the most common relations between the most popular and common business objects It does not cover each and every bit of the database but the most common bad highly usage one are covered. 

Secondly, if required you can generate diagram of a single table by displaying it in ABAP transaction dictionary (SE11).

By using the push button graphic you can make a lot out of it. But it is a tedious job.