I was confused with this too, but added the most common one to my slides when presenting my session; Getting Ready for Azure SQL Database. Is Azure SQL Database a Platform-as-a-Service, Database-as-a-Service or Software-as-a-Service?
First of all, let's try to understand these models. In addition to the ones mentioned, there is another model called IaaS - Infrastructure as a Service. Here is simple and shortened explanation on these models;
- IaaS - Infrastructure as a Service
When it comes to Cloud Computing, getting required Servers, Storage, Network as an on-demand service and maintenance of them is somone's else (like Azure, Amazon) responsibility, is called as IaaS. It is something like we have outsourced the entire IS service including physical components. Microsoft Virtual Machines in Azure is one of the examples for this.
- PaaS - Platform as a Service
This model sits on IaaS, offering a complete development and deployment environment. In addition to the services you get with IaaS, this offers services on Programming Languages, Database Services, etc. for making solutions. Microsoft Azure is a good example for this.
- SaaS - Software as a Service
This is the top layer of the Cloud Computing and it sits on PaaS, means on top of both IaaS and PaaS. This offers software required as a service on an on-demand way or as a subscription. Example, Gmail from Google that allows us to use email service just using the browser.
- DaaS - Database as a Service
This is not something much popular or not something always getting referred because it is an extension or an optimized SaaS model. This provides all RDMS functionalities with cloud computing and sits on PaaS.
This is how it looks like;
And this is the distribution of management;
Note sure the original author of second image, taken from one of Mircosoft pages
Now SQL Server is a PaaS, DaaS or SaaS? It is considers as PaaS but it is actually a SaaS. Can we consider it as a DaaS. Yes it is possible but I think that we should consider it as SaaS because it offers more that what traditional RDMS offers.
Here is an image taken from https://docs.microsoft.com/en-us/azure/sql-database/sql-database-paas-vs-sql-server-iaas