Case style of tables and field (Naming Conventions) What is best practice for naming dbs, tables (collections) and fields among DBMSs? 

I mean casing style, for example AaaBaa,aaa-bbb, aaa_bbb, AAA-BBB, aaaBbb, ... 

I think if one exists it should be applicable to all DBMSs  (MySQL, MongoDB, Oracle, ...), shouldn't it?

Currently I thing following is most appropriate:

Columns/Fields/Tables/Collections/Databases:  aaaBbbCcc.

Could distinct case style for these object be useful at all?
It all boils down to personal preference and consistency within your team (or just yourself). Nothing worse than seeing different styles mixed. 

As for SQL Server, be mindful of sp_yourProcName, as "sp_" prefix for a stored procedure can do funny things (read more about this from Aaron Bertrand). 

Consistency is king. Make your choice and stick with it.