As others above have suggested there are a number of issues to take into consideration when considering the development of a "database" application - whether just learning or not.
Some of the issues are (in no particular order).
* If this application will be for a company, then there is an issue of maintainability to consider. If you develop in a language which is obscure (relative to other market products) and there are only a few other developers in that language then the application can prove difficult to maintain should you be 'hit by a truck'.
* Are there expenses attached to the use of the database? Some databases have purchase prices and sometimes even licensing fees associated with them.
* What tools are available to use for developing user front ends. Databases such as MS SQL Server and others do not always have their own 'front end' development tools.
Yes, there are a whole host of other 'front end' development languages (
VB, Access, Visual Foxpro, and others) which can 'talk' to the SQL Server backend. But that might be an additional expense (or not). Its just something to consider.
* What about expandability? Will an application that you develop in language 'AAA' be able to 'grow' with enhancements, etc. to meet future needs or will it find itself dead-ended or obsoleted due to limitations? Again a question mostly concerned with applications developed for a business.
* If this is solely a personal learning endeavor, will the database tool(s) you choose support your learning a marketable set of skills or will it teach you a language which has minimal applicability and/or utilization in the marketplace?
In general a number of the mainstream Databases (MS SQL SERVER, INFORMIX, ORACLE, MYSQL, POSTGRESQL, etc.) aren't DRAMATICALLY different from each other in the generalities of what they can do and how you do it. Yes there are indeed differences, but in general they all can be accessed with ODBC, the data can be manipulated with SQL commands, etc.
The BIG differences you will encounter will be in the various Front End Development tools (many times not an integral part of the Database) - their language syntax and their capabilities at handling data manipulations.
From a strict marketability perspective I'd recommend using MS SQL Server Express as a database backend (its Free) and one of the .NET tools (
VB.NET or C#.NET) as a Front End development tool. This combination would not be the easiest to learn, but it would result in your learning very marketable skills. And if you were to develop an application, that application would have possible support from a wide variety of developers.
There are a number of other database development tools which are available and some have been mentioned above. They are OK for 'getting started' and the learning might be applicable for a number of years, but when you move up to the 'big time' you will have to re-educate yourself in one of the more 'mainstream' languages - some of them are VERY different from each other.
Good Luck