HailDB is a relational database in shared library form that can be embedded into applications. It is not a SQL database, although you can use this library as the storage backend for a SQL database. It features a clean and simple API and is a continuation of the Embedded InnoDB project. With the addition of the HailDB plugin to Drizzle, you get a full SQL interface.
fkchk is a MySQL InnoDB/XtraDB foreign key checker and repairer. It is useful if you suspect that your database's foreign keys may be broken, or if you disable foreign key checks and now want to be sure that the data is consistent. It is also useful if you have suffered a performance hit and want to check the consistency of foreign keys to verify that they're not the reason. It can attempt to repair any issues it finds, but this option should not be used and is only implemented for testing purposes.