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Gadfly
Where Java has a plethora of small memory-based relational databases such as McKoi or hSQldb, there is really only one contender for Python: Gadfly. Gadfly has some limitations: MySQL subqueries are not supported, for instance, and neither are inner or outer joins. Worse, it lacks the LIKE predicate. Still, it is a stable and useful tool in the toolbox, one that offers a good performance.