With MetaModel, you use a type-safe SQL-like API for querying any datastore. It is a data access framework providing a common interface for exploration and querying of different types of datastores. It isn't a data mapping framework. Instead, it emphasizes abstraction of metadata and the ability to add data sources at runtime, making MetaModel great for generic data processing applications, but less so for applications modeled around a particular domain.
jPersist is an extremely powerful object-relational database persistence API that manages to avoid the need for configuration and annotation; mapping is automatic (A-O/RM). It uses JDBC, and can work with any relational database and any type of connection resource. It uses information obtained from the database to handle mapping between the database and Java objects, so mapping configuration is not needed, and annotation is not needed. In fact, there is no configuration needed at all.
Itzam/C++ wraps the Itzam database engine with C++ templates for both simple and complex database classes. It supports B-tree, hash table, and sparse matrix indexes with unique or duplicate keys, and includes multithreaded resource locking and atomic file transactions. Complete class documentation is embedded in the source code, and available online and in the distribution archive.
QOF (Query Object Framework), provides a set of C Language utilities for performing generic structured complex queries on a set of data held by a set of C/C++ objects. This framework is unique in that it does not require SQL or any database at all to perform the query. Thus, it allows programmers to add query support to their applications without having to hook into an SQL database.