Pablo offers a great distinction of a data service vs. a database.
"...in a service you'll want the schema to be optimized for its target use, so semantics tend to drive it. In a database schema will be about data organization and performance."
A service is a different beast in this sense as well. In a service you just put the data "up there". The service will choose the appropriate physical organization for the data, regardless of the visible service schema. (for example, in the Astoria experimental online service you describe your data as entities and associations and the system figures out a logical/physical schema to support it, along with a mapping to translate between them). The system supporting the service may or may not use a relational database, or even a database at all (there are many large-scale storage systems that use other models instead of traditional relational to avoid the impact of global metadata and the complexity of partitioning highly structured schemas for scale-out).
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