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why should one use this over MySQL or something similar?
Like many NoSQL databases, one would use Redis if it fits your needs. It does not directly compete with RDBMS solutions like MySQL, PostgreSQL, etc. One may need to use multiple NoSQL solutions in order to replace the functionality of a RDBMS. I personally do not consider Redis to be a primary data store - only something to be used for speciality cases like caching, queuing, etc. Document databases like MongoDB or CouchDB may work as a primary data store and be able to replace RDBMSs, but there are certainly projects where a RDBMS would work better than a document database.