WHEN type=1 THEN payable_price WHEN type=2 THEN payable_price ELSE MAX(payable_price)

type=1 表示订单维度，type=2表示商品维度，需要无论type值为多少都取type=1时的payable_price值

解决方案：
使用SQL子查询;

SELECT  
    CASE   
        WHEN type = 1 THEN payable_price  
        WHEN type = 2 THEN (SELECT payable_price FROM your_table WHERE type = 1 LIMIT 1)  
        ELSE (SELECT MAX(payable_price) FROM your_table)  
    END AS amount  
FROM your_table;