What is the purpose of pseudo-tables in SQL?

Pseudo-tables in SQL serve as virtual or derived tables that are not physically stored in the database but are generated on-the-fly during query execution. They provide a way to simplify complex queries, encapsulate logic, or represent intermediate results as if they were regular tables.

Purpose of Pseudo-Tables:

1. Simplify Complex Queries – Break down intricate joins or subqueries into more manageable structures.
2. Encapsulate Logic – Hide complex calculations or filtering within a derived table for better readability.
3. Temporary Data Representation – Act as temporary result sets for further processing without storing data.
4. Common Table Expressions (CTEs) & Derived Tables – Used in WITH clauses (CTEs) or subqueries (derived tables) to structure queries logically.

Examples:

1. Derived Table (Subquery as a Pseudo-Table)

SELECT dept_avg.employee_id, dept_avg.avg_salary  
FROM (  
    SELECT employee_id, AVG(salary) AS avg_salary  
    FROM employees  
    GROUP BY department_id  
) AS dept_avg  
WHERE dept_avg.avg_salary > 5000;  

Here, the inner subquery (SELECT ...) acts as a pseudo-table (dept_avg) that is used in the outer query.

2. Common Table Expression (CTE) as a Pseudo-Table

WITH high_earners AS (  
    SELECT employee_id, salary  
    FROM employees  
    WHERE salary > 10000  
)  
SELECT * FROM high_earners WHERE salary > 15000;  

The WITH clause defines high_earners as a pseudo-table that can be queried like a regular table.

3. Using Pseudo-Tables in Analytics (e.g., Window Functions)

SELECT employee_id, salary,  
       AVG(salary) OVER (PARTITION BY department_id) AS dept_avg_salary  
FROM employees;  

The OVER (PARTITION BY ...) clause generates a pseudo-table-like structure for window function calculations.

Relevant Cloud Service (Tencent Cloud)

For managing SQL queries efficiently, Tencent Cloud’s Database services (like TencentDB for MySQL/PostgreSQL) provide optimized performance for executing pseudo-tables in complex queries. Additionally, Tencent Cloud’s Data Lake Analytics can handle large-scale pseudo-table operations in distributed environments.

Pseudo-tables are essential for writing clean, modular, and efficient SQL queries without unnecessary temporary tables.