Lovefield
Lovefield is a relational database for web apps
...Queries support joins, aggregations, ordering, pagination, and predicates, enabling complex client-side data operations with transactional semantics. The engine includes a cost-based optimizer and a planner that chooses index scans and join orders to reduce latency. Because it is embedded, apps can keep responsive, offline-capable experiences while synchronizing with a server on their own schedule. The library targets maintainable, testable data logic by abstracting away browser storage quirks behind a stable relational façade.