Which embedded database has maximum SQL compliance, and concurrency support?
I think it's Firebird.
Even Cte's are supported in Firebird 2.1 !
Firebird works on Windows, Linux, Mac OS...
Jaybird driver for Java.
There are some good migration tools from Access to Firebird:
- IBDatapump
- Microsoft Access to Firebird database converter (MDB2FDB)
I would take a look at the Advantage Database Server (sybase.com). It runs on Windows, Linux, and NetWare. We have a commercial accounting / project management application for the construction industry and switched to Advantage 6 or 7 years ago. We have been extremly pleased with the performance and support. Customers are very happy too.
You can find a fairly detailed description of Derby's SQL Standards compliance here: http://wiki.apache.org/db-derby/SQLvsDerbyFeatures