Microsoft SQL Server is a relational database management system (RDBMS) produced by Microsoft. Its primary query language is Transact-SQL, an implementation of the ANSI/ISO standard Structured Query Language (SQL) used by both Microsoft and Sybase.
The architecture of Microsoft SQL Server is broadly divided into three components: SQLOS which implements the basic services required by SQL Server, including thread scheduling, memory management and I/O management; the Relational Engine, which implements the relational database components including support for databases, tables, queries and stored procedures as well as implementing the type system; and the Protocol Layer which exposes the SQL Server functionality.
The Relational engine is the component, which implements the relational data store, using the capabilities provided by SQLOS, which is exposed to this layer via the private SQLOS API. It implements the type system, to define the types of the data that can be stored in the tables, as well as the different types of data items (such as tables, indexes, logs etc) that can be stored. It includes the Storage Engine, which handles the way data is stored on persistent storage devices, as well as implement methods for fast access to the data. The storage engine implements log-based transaction so as to ensure that any changes to the data are ACID compliant. It also includes the query processor, which is the component that allows data to be retrieved. The specification of what needs to be retrieved is provided in the form of a SQL query, which it optimizes and translates into the sequence of operations needed to retrieve the data. The operations are then scheduled on to the worker threads, which are scheduled for execution by SQLOS.