Comparison Operators

Lesson content

Comparison Operators Comparison Operators is a practical SQL concept used to design, query, secure, operate, or analyze relational data correctly. Example SELECT * FROM products WHERE price <> 0 AND stock >= 10; Key point Use the smallest correct statement, test it with representative data, and verify constraints and performance before production use. Real-life example A support dashboard needs a reliable list of matching orders without loading the entire orders table. Filters and ordering must produce the same result every time. Advanced example BEGIN; -- Preview the exact target set first. SELECT id FROM orders WHERE status = 'pending'; -- Apply the Comparison Operators operation, verify affected rows, then commit. COMMIT; 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 o.order_id, c.name AS customer, o.status, o.total FROM orders o JOIN customers c ON c.customer_id = o.customer_id WHERE o.total >= 1000 ORDER BY o.ordered_at 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 Comparison Operators: MySQL and PostgreSQL This operation is portable SQL in both MySQL and PostgreSQL. Confirm collation, case sensitivity, and NULL behavior with representative data because configuration can affect comparisons and ordering. 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.