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Most developers would describe a modern database as relational with stored procedures and cross-table functions such as join. So why would you use a database that has none of these capabilities? The answer is scalability.
This morning, CNN ran a story on your new web application. Yesterday you had 10 concurrent users, and now your site is viral with 50,000 users signing on. Which database will handle 50,000 concurrent users without a complex expensive cluster? The answer is SimpleDB.