Is there any additional database engine not standard one I’m using from the first?
Embedded Base databases, whether HSQL or Firebird store everything (tables, queries, forms, reports, macros plus a copy of the database engine) in a zip archive with the .odb extension. That is a handy format and a good learning tool, but if anything interrupts the zip process (often an impatient laptop user who closes the laptop while the database is in the process of being saved) the .odb file can become corrupt and the tables lost. Frequent backups are highly recommended when using an Embedded database.
The potential for data loss can be minimized by using a JDBC connection (sometimes called a split database) where the tables and database engine are stored outside of the .odb file. This also allows newer versions of the database engine to be easily installed.
[Wizard] Create a new ‘split’ HSQL 2.x database
The downsides are
In addition to the .odb file the database is composed of several other files in a directory with two sub-directories
Once created, tables cannot be edited with the Base table GUI but they can be edited with SQL commands. After becoming familiar with using SQL and armed with a handful of skeleton tables and other commands I’ve found that creating and editing the tables with SQL is usually easier than using the GUI.
The HSQL database engine is a mature and and well maintained product that closely adheres to the ISO standard for SQL. Various versions can be downloaded from https://sourceforge.net/projects/hsqldb/files/hsqldb/