Single instance

Single-instance databases are hosted as a single database on a single logical server. These databases do not have access to the complete features provided by SQL Server.

High availability

Azure SQL, by default, is 99.99% highly available. It has two different architectures for maintaining high availability based on the SKUs. For the Basic, Standard, and General SKUs, the entire architecture is broken down into the following two layers. There is redundancy built in for both of these layers to provide high availability:

• Compute layer
• Storage layer:

For the Premium and business-critical SKUs, both compute and storage are on the same layer. High availability is achieved by replication of compute and storage...