What are the Differences & Similarities among DB2, Oracle, SQL Server, MS Access?

Similarities:
Similarity is that they are all Relational Database, and they all are proprietary software. Otherwise there exist no similarity. All of them and developed/owned by companies like IBM, Oracle and Microsoft and work totally different ways towards the same need, that is storing organized relational data and retrieving them easily. 


Differences: 
All have different user interfaces and use different scripting languages (Oracle - PL/SQL, SQL Server - T/SQL, Access - VBA, etc.). Most (if not all) also have a few add-ons to SQL (built-in functions and the like).

Information on DB2 certification

well there are 3 levels in DB2 certification exam. associate(entry); Administrator/Developer(intermediate) and Administrator(Advanced) levels. Currently IBM DB2 9 version is working in IT industry.

At the entry level there is Exam 730 and the intermediate there are two exams. it depends on the candidate which exam he/she want to give. 1st exam is 731, which is on LUW (Linux, Unix and Windows) and z/OS. Advanced level exam 734 is the last exam in IBM DB2 9 certification list.