Subqueries

Lesson content

Subqueries Subqueries is a practical SQL concept used to design, query, secure, operate, or analyze relational data correctly. Example SELECT * FROM products WHERE price > (SELECT AVG(price) FROM products); Key point Use the smallest correct statement, test it with representative data, and verify constraints and performance before production use. Real-life example Customer, order, and product data live in separate tables. A report must combine them while retaining the correct rows and avoiding accidental duplicates. Advanced example WITH customer_totals AS ( SELECT customer_id, SUM(total) AS spent FROM orders WHERE status = 'paid' GROUP BY customer_id ) SELECT c.id, c.name, t.spent FROM customers c JOIN customer_totals t ON t.customer_id = c.id WHERE t.spent > 1000 ORDER BY t.spent DESC; Expected result The query returns only the intended rows and columns, with deterministic ordering where order matters. Production check Test with empty, duplicate, null, and boundary values. Use a transaction for related writes. Inspect the execution plan before adding an index. Use parameterized queries for application input. Continue with the PicoStore database This lesson reuses picostore . Relevant tables: customers, products, orders, order_items . Keep the starter rows from the Introduction lesson so results remain comparable. Another practical example SELECT c.name, COUNT(o.order_id) AS order_count, COALESCE(SUM(o.total), 0) AS lifetime_value FROM customers c LEFT JOIN orders o ON o.customer_id = c.customer_id GROUP BY c.customer_id, c.name ORDER BY lifetime_value DESC; Check the result Run the verification query, compare the returned rows with the starter data, and explain why every included or excluded row is correct. Easy example Start with a small customer table and retrieve active customers in a predictable order. SELECT customer_id, name, email FROM customers WHERE status = 'active' ORDER BY name; How to verify the easy example Run it with representative input. Confirm the expected output. Try one missing, invalid, or boundary value. Advanced example Use a CTE and a window function to rank customer revenue while keeping the query readable and testable. WITH customer_revenue AS ( SELECT customer_id, SUM(total_amount) AS revenue FROM orders WHERE order_status = 'completed' GROUP BY customer_id ) SELECT customer_id, revenue, DENSE_RANK() OVER (ORDER BY revenue DESC) AS revenue_rank FROM customer_revenue ORDER BY revenue_rank, customer_id; Advanced review Explain the tradeoffs and assumptions. Test failure, scale, security, and recovery behavior. Capture evidence from tests, execution plans, logs, or review output. Additional practical guidance Subqueries: MySQL and PostgreSQL The relational idea works in both MySQL and PostgreSQL. Verify duplicate handling, NULL behavior, and the execution plan; vendor support differs for some set operators, recursive syntax, and materialization choices. Required verification Run the simple case. Test a NULL, duplicate, empty, or boundary case where relevant. Confirm the affected rows or query result. Use EXPLAIN for performance-sensitive queries.