What difficulties can arise when using nested SELECT queries in SQL?

When using nested SELECT queries (subqueries) in SQL, the following difficulties may arise:

• Performance degradation: nested queries can execute more slowly, especially if subqueries are not optimized or do not use indexes.
• Complexity of reading and maintaining code: nested queries can be hard to read and debug.
• Functional limitations: some DBMSs do not support certain types of subqueries or have restrictions on their use.
• Caching issues: subqueries can interfere with effective cache utilization.

For example, instead of a nested query, you can use JOIN, which is often more efficient:

-- Nested query
SELECT * FROM orders WHERE customer_id IN (SELECT id FROM customers WHERE status = 'active');

-- Alternative with JOIN
SELECT o.* FROM orders o JOIN customers c ON o.customer_id = c.id WHERE c.status = 'active';