@pbj, you are free to never visit this site again. Given you've suggested as much, and seem to not respect advice given (in fact take it as an attack), it doesn't give much incentive to answer.
Be that as it may, and at the risk of this never being read or sounding 'elitist': my advice would be to skip JDBC for the moment. Become very familiar with SQL first. Learn about database normalization, how to manage a database (database files, schemas/databases, users, security, etc...), how to construct queries, joins, unions, indexing, primary keys, foreign keys and referential integrity, etc...any database will suffice (Oracle, MySQL, PostgreSQL, etc...) - while each has its intricacies the basics are fairly constant from one to the next (yes, which one you choose matters - some hosts only support certain systems, some are not amenable to embedding into an application, some have larger memory footprints, some have better procedural support, and some provide commercial support). I would not advise choosing your database because it is 'easiest' - you may end up being limited in other ways which force you to use the 'harder' one. PostgreSQL and MySQL are popular, free, and IMO good intro systems to learn the necessary subjects. Once familiar with SQL and database administration then move into JDBC, the transition should be much more smooth.
I'm afraid you won't find a friendlier place. I was suspended from Stack Overflow for asking questions people found annoying. I asked the same here, and was greeted with friendly faces. And with that attitude you won't go anywhere.
Database is always a headache for me and I am not into it since the beginning because the complicated logic and so much data for computing. Then I started seeking a easy way to do the complex data computing, a very small tool named esProc can easily work out a difficult database computing issue. Database is really important for us, but sometimes we really feel frustrated!:o
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I assume that this advice would be very helpful for pbj because I read and think very word of it. Learning the basics about database administratin and SQL before moving into JDBC would be a very good idea, but it may take some time to learn every aspect very well. Here may I ask how to compute the data which are from different datasources(example: one data source is Oracle, the other is from SQL Server or Excel sheet, how to combine both and do some computation? Thanks!)