Azure SQL or Azure Cosmos DB?
Azure SQL is a family of services on the SQL Server engine for relational data: Azure SQL Database, Azure SQL Managed Instance and SQL Server on Azure VMs. Azure Cosmos DB is a fully managed NoSQL and vector database built for global distribution, elastic scale and flexible schemas. The data model usually decides.12
How they relate
Comparable. They can be alternatives for some goals; which one fits depends on the goal.
Azure SQL is not a single service but a family that runs the SQL Server engine in Azure: Azure SQL Database (PaaS, with Hyperscale, serverless and elastic pool options), Azure SQL Managed Instance (PaaS with almost full SQL Server feature parity, which Microsoft recommends for most migrations), and SQL Server on Azure VMs (IaaS with operating system access). Azure Cosmos DB is a fully managed NoSQL and vector database that holds document, key-value, graph and table data and offers turnkey multi-region writes.
Microsoft's own guidance splits them by workload. The Cosmos DB documentation lists flexible schemas, latency-sensitive, highly elastic, high-throughput and highly available mission-critical apps as good fits, and names highly relational apps as a poor fit, pointing to Azure SQL or Azure Database for MySQL instead. Cosmos DB offers a 99.999% availability SLA for multi-region accounts. Azure SQL keeps the relational model and familiar SQL Server tools, with SQL Database aimed at modern cloud apps and Managed Instance at moving existing SQL Server estates with minimal changes.
Both can feed analytics: Cosmos DB and Azure SQL Database can be mirrored into Microsoft Fabric's OneLake, and Microsoft advises against using Cosmos DB for analytical (OLAP) workloads, pointing to Fabric. Cosmos DB also stores embeddings next to operational data with integrated vector search. You may need neither: for MongoDB-compatible workloads Microsoft offers Azure DocumentDB, a separate vCore service, and for open-source relational engines Azure Database for PostgreSQL or MySQL.1234
The technologies
Cloud service
Azure SQL
Azure SQL is Microsoft's family of SQL Server engine services on Azure: Azure SQL Database for cloud applications (with Hyperscale and serverless options), Azure SQL Managed Instance for near-complete SQL Server compatibility, and SQL Server on Azure Virtual Machines for full OS and engine control.
Cloud service
Azure Cosmos DB
Azure Cosmos DB is a fully managed NoSQL and vector database for applications that need low latency, elastic scale and global distribution. It offers multi-region writes with a 99.999% availability SLA, built-in vector and hybrid search, and serverless or provisioned throughput billed in request units.
Which fits which goal
| If your goal is | What fits | Why |
|---|---|---|
| Build a new relational cloud app with transactions and joins | Azure SQL | Microsoft positions Azure SQL Database for modern cloud applications on a managed SQL Server engine, and the Cosmos DB documentation names highly relational apps as a poor fit for Cosmos DB.12 |
| Move an existing SQL Server estate to Azure with minimal changes | Azure SQL | Azure SQL Managed Instance offers almost full feature parity with the SQL Server engine and is Microsoft's recommended target for most migrations; SQL Server on Azure VMs covers cases that need OS-level access.1 |
| Serve users worldwide with low latency and multi-region writes | Azure Cosmos DB | Cosmos DB provides turnkey global distribution and multi-region writes with automatic failover, backed by a 99.999% availability SLA for multi-region accounts.2 |
| Store application data whose schema changes often | Azure Cosmos DB | Microsoft lists flexible schema for iterative development as a good fit for Cosmos DB, which indexes all data automatically regardless of the data model.2 |
| Keep AI agent memory and embeddings next to operational data | Azure Cosmos DB | Cosmos DB integrates vector and hybrid similarity search so embeddings can sit with operational data for RAG, AI agents and LLM caching.2 |
| Run a SaaS app with many tenant databases of varying load | Azure SQL | Elastic pools in Azure SQL Database share resources across databases with variable usage and are optimized for the multitenant SaaS application pattern.1 |
No technology here is ranked. Pricing and performance are left out on purpose: they depend on your models, hardware and contract, so measure them in a pilot.
Sources
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