Azure SQL vs. Azure Cosmos DB?

Which database is the better choice, Azure SQL or Azure Cosmos DB? 

Migrating to the cloud is no longer a question of if, but when and how. Your data architecture will determine your product’s cost, scalability, security and pace for years to come. Within the Azure environment, the two key services most often compared are the relational Platform as a Service (PaaS) Azure SQL Database service and the multi-model, globally distributed Azure Cosmos DB. These services differ in their approach to data modelling, consistency, scaling and billing. This article will help you quickly identify which technology better supports your application scenario, saving you from investing in costly migrations or refactorings. 

Get to know Azure SQL

Let’s start by looking at the characteristics of Azure SQL Database.  It is a fully managed, relational Platform as a Service (PaaS) offering based on the SQL Server engine and Transact-SQL (TSQL)-compliant, enabling you to easily migrate existing .NET, Java or Python applications that use the SQL ecosystem. It provides ACID transactionality, referential integrity, rich indexing (including Columnstore) and programmability through procedures, functions and triggers. Active GeoReplication enables the asynchronous maintenance of up to four readable copies in other regions and facilitates fast failover. Transparent Data Encryption (TDE) encrypts data ‘at rest’, while Always Encrypted protects sensitive columns with client-side encryption. Integration with Microsoft Entra ID simplifies centralised identity control and multi-factor authentication (MFA). Scaling is done on a vCore (a virtual core, or logical CPU bound by memory) or a DTU (Database Transaction Unit, or a combined measure of CPU, memory and I/O) model. Analytics integrations include Azure Synapse Link for SQL, which provides near-real-time replication of operational data without ETL, and Microsoft Fabric mirroring for streaming data in OneLake. 

How does Azure Cosmos DB work? 

Azure Cosmos DB is the second offering from Microsoft. It is a fully managed, globally distributed, enterprise-class NoSQL database designed for low latency and flexible horizontal scaling. It supports multiple models within a single service: document (SQL/NoSQL API, JSON), MongoDB, Cassandra (columnar), Gremlin (graph) and Table (key-value). Data is automatically partitioned and bandwidth is declared in RU/s (request units per second). Autoscale mode can also be used. Enabling multiregion writes provides regional replication and five levels of consistency (Strong, Bounded Staleness, Session, Consistent Prefix and Eventual) enable you to balance latency and guarantees. Security is strengthened by AES256 encryption, Private Link and RBAC roles with Microsoft Entra ID. Synapse Link for Cosmos DB and Fabric mirroring integrations make HTAP scenarios and near-real-time analytics available without the need for costly export. 

Key differences between Azure SQL and Azure Cosmos DB 

Which database is the best choice? 

Azure SQL is a good option: 

• If your data has many relationships and requires complex joins, it will be easier (and cheaper) to model it in Azure SQL. In Cosmos DB, however, data denormalisation is likely to be required, 
• You are building a finance and accounting or ERP system where strong transactions, relationships, and complex reports are essential, 
• Your team uses the SQL Server ecosystem and requires minimal training, 
• Vertical scaling (up to 128 vCores) in one region is sufficient for the expected traffic, 
• Regulations (e.g. SOX, PSD2 and HIPAA) require precise schema control and columnar encryption. 

Choose Azure Cosmos DB when: 

• You are running a global e-commerce shop that requires a TTFB of less than 200 ms for customers on all continents, 
• You are collecting billions of IoT events and need to dynamically add partitions without migration, 
• You are creating a mobile game with rankings and savegames that are replicated to multiple regions so that players don’t experience lag, 
• You don’t need to build your own replication system, as is required with Azure SQL. 
• You are storing documents with heterogeneous schemas (e.g. headless CMS, user profiles) and only want to pay for RU/s consumed.

So, what is it like with this option? 

There is no single ‘better’ database; rather, the right choice depends on your data model, performance requirements, geographic reach, regulatory requirements and cost profile. Azure SQL simplifies migration from existing relational systems and provides a rich ecosystem of tools, ACID transactionality and enterprise-class security features, such as Transparent Data Encryption (TDE) and Always Encrypted. Azure Cosmos DB, on the other hand, provides global distribution, linear scaling and an elastic schema, as well as five levels of consistency, which accelerates the construction of modern, multi-region applications with variable workloads. In practice, many architectures combine the two: transactional data is stored in Azure SQL and event, profile or cache data is stored in Cosmos DB, which is analysed further in Synapse or Fabric. Ultimately, you need to assess access patterns, required SLAs, expected data growth rates and budget, and then make an informed decision.