Poke around the infrastructure of any startup website or mobile app these days, and you're bound to find something other than a relational database doing much of the heavy lifting. Take, for example, the Boston-based startup Wanderu. This bus- and train-focused travel deal site launched about three years ago. And fed by a Web-generated glut of unstructured data (bus schedules on PDFs, anyone?), Wanderu is powered by MongoDB, a "NoSQL" database—not by Structured Query Language (SQL) calls against traditional tables and rows.