UNION and UNION ALL

Lesson content

UNION and UNION ALL UNION and UNION ALL is a practical SQL concept used to design, query, secure, operate, or analyze relational data correctly. Example SELECT email FROM customers UNION SELECT email FROM suppliers; 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 schema or operation satisfies the stated rule, rejects invalid data, and can be verified with a repeatable query. 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 UNION and UNION ALL: 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.