I will remove @author tags from SQL92 implementation to follow Mahout standard. For the record, I think the Apache coding guideline (http://portals.apache.org/development/code-standards.html) suggests using the @author tags.
I agree not to bother with MySQL. Between Postgres and SQL92, I'd rather have SQL92 as base. Because SQL92 is not likely to change in the future, but Postgres might. Especially if you look at PostgreSQLJDBCDataModel.java line 62:
private static final String POSTGRESQL_DUPLICATE_KEY_STATE = "23505"; // this is brittle...
This is probably going to change in the future, and might affect SQL92 if SQL92 derives from Postgres...
On the other hand, the benefit of duplicated code is that it allows some independence. Besides, the only duplication between SQL92 and Postgres is with the constructors (and the fact that they both override setPreference(), though differently). It has some but not much benefit to have Postgres extends SQL92 or vice versa. So I think the current implementation is fine too.
The reason I'd like to push SQL92 is that Drupal 7 supports Oracle/MSSQL/SQLite (http://drupal.org/requirements). So I need to make sure Mahout can access those databases too for Drupal/Mahout integration.
What do you think?