CASE WHEN 是 SQL 查詢中的一個條件表達式，它允許你根據一個或多個條件返回不同的值

以下是 CASE WHEN 在 SQL 查詢中的一些實際應用：

1. 計算學生成績等級：

SELECT
    student_id,
    student_name,
    score,
    CASE
        WHEN score >= 90 THEN 'A'
        WHEN score >= 80 THEN 'B'
        WHEN score >= 70 THEN 'C'
        WHEN score >= 60 THEN 'D'
        ELSE 'F'
    END AS grade
FROM students;

2. 根據員工的工作年限分類：

SELECT
    employee_id,
    employee_name,
    years_of_experience,
    CASE
        WHEN years_of_experience < 3 THEN 'Junior'
        WHEN years_of_experience >= 3 AND years_of_experience < 5 THEN 'Intermediate'
        ELSE 'Senior'
    END AS experience_level
FROM employees;

3. 根據客戶的消費金額分類：

SELECT
    customer_id,
    customer_name,
    total_spending,
    CASE
        WHEN total_spending >= 10000 THEN 'VIP'
        WHEN total_spending >= 5000 AND total_spending < 10000 THEN 'Regular'
        ELSE 'New Customer'
    END AS customer_type
FROM customers;

4. 根據訂單狀態顯示不同的信息：

SELECT
    order_id,
    order_date,
    order_status,
    CASE
        WHEN order_status = 'Shipped' THEN 'Your order has been shipped.'
        WHEN order_status = 'Processing' THEN 'Your order is being processed.'
        WHEN order_status = 'Cancelled' THEN 'Your order has been cancelled.'
        ELSE 'Your order status is unknown.'
    END AS order_message
FROM orders;

這些示例展示了如何使用 CASE WHEN 在 SQL 查詢中根據不同的條件返回特定的值。這可以幫助你根據數據庫中的數據創建更復雜的查詢和報告。