Can you explain the main principles and mechanisms of executing clean SQL queries in a database?

A pure SQL query is an instruction that is directly executed by the DBMS to work with data. The main principles:

• Queries are written in SQL language and can be of different types: SELECT, INSERT, UPDATE, DELETE.
• When executed, the DBMS first parses the query, checking syntax and semantics.
• Then, an execution plan is built, and the optimizer chooses the most efficient way to access the data.
• After that, the query is executed, and data is retrieved or modified.
• The result is returned to the client.

For example, a simple SELECT query:

SELECT id, name FROM users WHERE active = 1;

The DBMS will find all active records in the users table and return them. It is important to note that a pure SQL does not contain application logic — it only describes data operations.