Re: difference between all RDBMSs

Obviously on this list you will mostly get info on PostgreSQL.  With 
regard to PostgreSQL, I would highly suggest familiarizing yourself with 
the online documentation.  I won't cover the weaknesses of MySQL here, 
but will give you a quick overview on how PostgreSQL is different from 
other RDBMS's so you can refine your search a bit.

PostgreSQL is designed to be extremely extensible.  This means that one 
can easily write code to add data types, procedural languages, and more 
with very little work.  Other database managers may allow for data types 
to be added, but I am not aware of any others that allow you to define 
your own procedural langauges in any arbitrary way (Even the recent 
enhancements to MS SQL to give it access to .Net are not this advanced).

The PostgreSQL development team has made data integrity and stability 
(assuming working hardware) a top priority.  It is certainly a higher 
priority than any other open source RDBMS I have ever worked with.

If you want to understand other factors that make PostgreSQL different 
than other RDBMS's you may want to look into differences regarding:
ISO compliance (which features of SQL-99 are supported), the trigger vs. 
rule systems in PostgreSQL (warning MS SQL uses something they call 
rules but it is something different), and features like inherited 
tables.  Again, read the online documentation.

Best Wishes,
Chris Travers
Metatron Technology Consulting