What methods are used to split complex queries into parts in PostgreSQL?

In PostgreSQL, the following methods are often used to break down complex queries into parts:

1. CTE (Common Table Expressions) — WITH expressions allow you to split a query into logical parts that can be reused within the main query.

WITH recent_orders AS (
    SELECT * FROM orders WHERE order_date > CURRENT_DATE - INTERVAL '30 days'
)
SELECT customer_id, COUNT(*) FROM recent_orders GROUP BY customer_id;

2. Subqueries — nested SELECT statements within the main query for step-by-step filtering or aggregation.

3. Functions and procedures — part of the logic can be moved to user-defined functions in PL/pgSQL or other languages.

4. Temporary tables — creating temporary tables for intermediate results, especially if queries are very heavy and repeated.

These methods help improve readability, reusability, and optimization of complex queries.