How can you create and optimize the execution of an SQL query considering its structure and order of operations?

To create and optimize an SQL query, it is important to understand the order of execution of operators in SQL, which is usually as follows: FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY.

Optimization includes:

• Selecting the necessary columns instead of SELECT * to reduce data volume.
• Using indexes on columns involved in JOIN, WHERE, and ORDER BY.
• Filtering data as early as possible (in the WHERE clause) to reduce the amount of data processed.
• Avoiding subqueries when possible, replacing them with JOINs to improve performance.
• Checking the execution plan (EXPLAIN PLAN) to identify bottlenecks.

An example of a simple optimized query:

SELECT employee_id, name
FROM employees
WHERE department_id = 10
ORDER BY name;

Here, filtering by department_id occurs before sorting, which reduces the data volume for sorting.