Amazon RDS for Microsoft SQL Server offers cloud instances of SQL Server. Amazon’s offering even supports “License Mobility”, Microsoft jargon that lets volume licensing customers use an existing SQL Server license for an Amazon’s instance. But how does Amazon’s cloud SQL Server compare with Microsoft’s own offering, SQL Database running on Azure?

Peter Marriott has posted on the subject here (registration required). The key point: despite the obvious similarity (both are SQL Server), these two offerings are radically different. Amazon’s RDS SQL is more IaaS (infrastructure as a service) than PaaS (platform as a service). You choose an edition of SQL Server and rent one or more instances. The advantage is that you get full SQL Server, just like the on-premise editions but hosted by Amazon.

Microsoft’s Azure-hosted SQL on the other hand is more abstracted. You do not rent a SQL Server instance; you rent a database. Under the covers Microsoft provides multiple redundant copies of the data, and if traffic increases, it should scale automatically, though the database size is limited to 150GB. The downside is that not all features of SQL Server are available, as I discovered when migrating data.

Marriott adds that SQL Azure supports encrypted connections and has a more usable administration interface.

A further twist: you can also install SQL Server on an Azure Virtual Machine, which would get you something more like the Amazon approach though I suspect the cost will work out higher.