Distributed storage systems support SQL while also providing scalability and fault tolerance by utilizing an SQL library (the SQL execution component) layered on top of a transactional key-value system (the storage component). The SQL library comprises a parser and an execution engine running at the client, while the storage component comprises a key-value system for storing the data. The SQL library parses SQL queries received from client applications and maps them to transactions on the key-value system. The key-value system stores data items identified by key and provides concurrency control mechanisms to handle concurrent transactions. The key-value system may be a distributed system with several storage servers, each attached to one or more storage devices.