To create any non-trivial application, you need some way of persisting data. While there are a lot solutions available, Datomic, Neo4j, Samza, just to name a few, but for various reasons, a lot of us still use relational databases. One reason might be because there is a pre-existing Rails application that still needs to interoperate with the Clojure application you are building. Another reason could be because of the exisiting deployment environment requires you to use a certain type of database (I encountered this first hand while working for a large client).
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| | | |
| Ruby | | |
| on | | Compojure |
| Rails | | (Clojure app) |