The CASE WHEN statement is a fundamental SQL construct for conditional logic. It enables you to return different values based on the evaluation of conditions within a query. This is particularly useful for transforming data or creating calculated fields based on specific criteria. Imagine you have a table of customer orders, and you want to categorize them as 'High-Value', 'Medium-Value', or 'Low-Value' based on the order total. The CASE WHEN statement makes this straightforward. It's a versatile tool that can be used in SELECT, UPDATE, and other SQL statements. The basic structure involves a series of WHEN conditions followed by a THEN clause specifying the value to return if the condition is true. A final ELSE clause (optional) handles cases where none of the WHEN conditions are met. This allows for complex conditional logic within a single query, enhancing the flexibility and power of your SQL operations.