使用复杂case-when语句的SQL查询

SELECT column_name,
       CASE
           WHEN condition1 THEN result1
           WHEN condition2 THEN result2
           ...
           ELSE result
       END
FROM table_name;

SELECT order_id,
       CASE
           WHEN status = 'pending' THEN '待处理'
           WHEN status = 'processing' THEN '处理中'
           WHEN status = 'completed' THEN '已完成'
           ELSE '未知状态'
       END AS status_desc
FROM orders;

SELECT order_id,
       CASE
           WHEN amount < 1000 THEN '小额订单'
           WHEN amount >= 1000 AND amount < 5000 THEN '中等订单'
           WHEN amount >= 5000 THEN '大额订单'
           ELSE '未知金额'
       END AS order_category
FROM orders;

SELECT user_id,
       SUM(CASE
               WHEN status = 'completed' THEN amount
               ELSE 0
           END) AS total_amount
FROM orders
GROUP BY user_id;