Not sure what you mean...
Adabas C is an inverted-list DBMS with a well-deserved reputation as a very fast and reliable, although non-relational, database. Nothing to stop you making your table design relational but all the integrity handling, constraints, triggers etc. would have to be performed by the application itself.
Natural is a 4GL development language.
Both come from Software AG and are generally, but not always, used together. Natural's own source and objects, security, cross-referencing data etc are themselves stored on Adabas files (on the mainframe anyway).
Adabas can be accessed from most 3GL languages (including COBOL) using direct calls. Can also be accessed via SQL using an add-in (although since Adabas is NOT a relational DB the translation and/or performance can be a bit problematic).
Natural is usually written to access Adabas using its own verbs (READ, FIND, GET etc which are internally translated into calls) although direct calls can be used. Can also access normal flat files, VSAM files and SQL databases.
If you're a COBOL programmer you'll have no problem with the overall feel and syntax of Natural.
BUT...if you're accustomed to SQL and now need to use Natural's own data access commands (READ, FIND etc) there will be a learning curve since the database concepts are different and, as a Natural programmer, you have a LOT more control over how the data is retrieved.
Adabas D is a completely separate product, a true relational/SQL database.