At the time of writing, there are three DTU service tiers for Azure SQL Database: Basic, Standard, and Premium. All of these offer support for elastic database pools and single databases only, but not the SQL database managed instance. The performance of these tiers is expressed in Database Transaction Units (DTUs) for single databases, and elastic Database Transaction Units (eDTUs) for elastic database pools.
DTUs specify the performance for single databases, as they provide a specific amount of resources to that database.
On the other hand, eDTUs do not provide a dedicated set of resources for a database, as they share resources within a specific Azure SQL Server with all the databases which run that server.
For more information about DTUs and eDTUs, you can check out the following article: https://docs.microsoft.com/en-us/azure/sql-database/sql-database-what-is-a-dtu.
To calculate your required DTUs, especially when you are migrating an on-premises SQL Server database, you can use the Azure SQL DTU calculator, which can be accessed at the following link: http://dtucalculator.azurewebsites.net/.
The following is a table from Microsoft that illustrates the different tiers' performance levels for elastic database pools:
| Basic | Standard | Premium | |
| Maximum storage size per database | 2 GB | 1 TB | 1 TB |
| Maximum storage size per pool | 156 GB | 4 TB | 4 TB |
| Maximum eDTUs per database | 5 | 3,000 | 4,000 |
| Maximum eDTUs per pool | 1,600 | 3,000 | 4,000 |
| Maximum number of databases per pool | 500 | 500 | 100 |
The following illustrates the different tiers' performance levels for single databases:
| Basic | Standard | Premium | |
| Maximum storage size | 2 GB | 1 TB | 4 TB |
| Maximum DTUs | 5 | 3,000 | 4,000 |