The Microsoft SQL Database Engine

The Microsoft SQL Database Engine is the core service for storing, processing, and securing data. It provides controlled access and rapid transaction processing for a wide variety of commercial and entrepreneurial applications.

The SQL Database Engine is the common underlying engine behind all Microsoft SQL offerings. It's an enterprise-scale, general-purpose relational database management system (RDBMS).

For millions of customers worldwide, in every industry and level of organization, the Microsoft SQL Database Engine is the database service for secure data processing and storage. Demanding applications can reliably read and modify information while preserving integrity at scale. The SQL Database Engine runs as a service that accepts client connections and then executes the requested operations against databases. Secured with enterprise-class data access and encryption features, and with built-in high availability and database recovery features, the SQL Database Engine is a complete database platform.

Multi-model database engine

The SQL Database Engine is a multi-model database engine with purpose-built storage formats, purpose-built index structures, and a single query optimizer that makes cost-based decisions across all data models.

Products that use the SQL Database Engine can be the enterprise-class online transactional processing (OLTP), online analytical processing (OLAP), or non-relational solution for your modern applications. Relational, normalized data might be common for databases, but the SQL Database Engine supports many data models and data formats.

All capabilities appear in the same SQL Database Engine, using the same Transact-SQL (T-SQL) query language, under the same security layer, and using the same HA/DR solutions.

• Columnstore indexes: overview
• Graph processing with SQL Server and Azure SQL Database
• Key-value pair
• JSON data in SQL Server
• Spatial Data
• Vector search and vector indexes in the SQL Database Engine
• XML data (SQL Server)

Tooling

The SQL Database Engine comes with a suite of free, industry-leading tools for querying, data architecture, automation, and database development.

Database fundamentals: ACID compliance

A core tenet of any RDBMS is support for ACID properties of transactions. A transaction is a sequence of operations performed as a single logical unit of work. A logical unit of work must exhibit four properties to qualify as a transaction: atomicity, consistency, isolation, and durability (ACID).

For more information on transactions, see Transaction locking and row versioning guide. For a deep dive into the SQL Database Engine, see SQL Server internals and architecture guides.

By design and by default, the SQL Database Engine is a fully ACID compliant database. In the interest of scale or performance, database developers can intentionally bypass some ACID principles in the SQL Database Engine. For example, they can use delayed durability, non-durable tables, or read uncommitted data. In all these cases, the developer makes a choice to trade off some ACID properties to achieve other goals. Such tradeoffs must be made with caution because they can result in data integrity issues and affect business outcomes.

Platform feature support

Most Transact-SQL (T-SQL) features that applications use are fully supported on all SQL Database Engine platforms. For example, core SQL components such as data types, operators, and string, arithmetic, logical, and cursor functions work identically in all platforms. However, there are a few T-SQL differences in data definition language (DDL) and data manipulation language (DML) elements. These differences result in T-SQL statements and queries that are only partially supported in various platforms for design reasons.

In the case of platforms as a service (PaaS) or software as a service (SaaS) platforms, for example, some operating system or local file capabilities are disabled due to logical or physical isolation. In a contained database, for example, T-SQL statements and options aren't available if they configure instance-level options, operating system components, or specify file system configuration.

Features and links

The following table lists major features of the SQL Database Engine with links to overview documentation and brief descriptions.

Language and driver support:

Modern platforms using the SQL Database Engine

The following modern platforms use the SQL Database Engine, starting with the flagship SQL Server product.

In SQL documentation, the Version selector dropdown list is key to understanding which version applies to an article, syntax reference, tutorial, or other content. Many Learn articles are customized to fit specific SQL platforms and capabilities. In most SQL reference articles, there's also an icon bar listing the applicable platforms for an article. For more information about navigating documentation, see SQL Server docs navigation guide.

Free offers

Get started today, for free. You can try the SQL Database Engine for free, with a free Azure subscription:

• SQL Server Developer editions for development and test systems
• Azure SQL Database free tier
• Azure SQL Managed Instance free offer
• Microsoft Fabric for free trial capacity

Migration

The Azure Database Migration Guides landing page provides links to quickly start migrations from various platforms to various Microsoft SQL platforms.

• Compare SQL data migration tools for a wide variety of migrations.
• Visit the Microsoft Fabric migration overview to learn more about migration to Fabric.
• Try out the Azure Database Migration Service (Azure DMS), a fully managed service for migrations from multiple database sources to Azure data platforms.

Related content

• What is SQL Server?