Oracle/DB

A database management system is a computer software application that interacts with the user, other applications, and the database itself to capture and analyse data. A general-purpose DBMS is designed to allow the definition, creation, querying, update, and administration of databases. DBMS can be used to manage transactional databases, such as HR systems, banking systems and so on. Database administrators (DBAs) use specialised software to store and organise data. The role may include capacity planning, installation, configuration, database design, migration, performance monitoring, security, troubleshooting, as well as backup and data recovery. You can do Oracle DBA course and certification after 12th but the chances of getting a job in medium or large size jobs are very poor unless you complete minimum graduation degree such as BBA. It is my sincere advise to you to pursue graduation degree first, else you will face challenges and disappointments while searching for a job.

The following information might help you to decide. Oracle is not just a database provider, although, they are known for the database. They also have Oracle ERP, Oracle E-Business Suite Financials, etc.
On the other hand, SAP is not only a vendor for enterprise software such SAP ECC (previously known as SAP R/3) or SAP Business One. They now also have SAP HANA - in-memory database.
So you see, from the ground up both companies are alike. They both have databases (SAP only has SAP HANA in memory) and both have ERP software (among other solutions).
I for a fact know people that came from Oracle Financials into SAP Financials and vice-versa. The trick is to understand the business processes and get skills that are easily transferable from one technology to the other.
By the end of the day, a receivable is still a receivable regardless if you use SAP or Oracle.
Ps: Mind that a lot of SAP installations still run on top of Oracle databases. With SAP Hana, it is not certain that the landscape will remain unchanged in the next few years.