SQL Query Processing - Microsoft Learn

Introduction to SQL Query Processing

SQL query processing is the process of transforming a logical query into an executable plan that the database engine can understand and execute.

This topic covers the key steps involved in query processing, including:

Parsing

The parser examines the SQL query and verifies its syntax.

If the syntax is invalid, the parser generates an error message.


        SELECT *
        FROM Customers
        WHERE City = 'London'

Semantic Analysis

The semantic analyzer checks the validity of the query based on the database schema.

This includes checking if the tables and columns exist, and if the data types are compatible.

Query Rewriting

The query is rewritten to improve its efficiency.

This can involve applying rules to simplify the query or convert it to a more efficient form.

Optimization

The query optimizer generates an execution plan that minimizes the resources required to execute the query.

This involves considering various factors, such as indexes, data size, and system load.

Execution

The execution plan is executed to retrieve the data from the database.

This involves sending commands to the database engine to retrieve the data.