选择不产生价值后的情况

Question

表

SELECT o.receipt, COALESCE(r1.amount, r2.amount, r3.amount) as amount
FROM   ORDERS as o
   LEFT JOIN RECEIPT1 as R1 ON o.receipt = r1.receipt and o.order_type = 'c'
   LEFT JOIN RECEIPT2 as R2 ON o.receipt = r2.receipt and o.order_type = 'x'
   LEFT JOIN RECEIPT3 as R3 ON o.receipt = r3.receipt and o.order_type = '??'

查询

Type   |  Name   |   Condition
Fruit  |  Apple  |    Fair
Fruit  |  Apple  |    Fair
Veg    |   null  |    Good
Veg    |  Tomato |    Great

尝试让案例显示重复项和空白的值。我想要生成为“空白”的第二个WHEN语句在运行语句后不会生成值。这似乎只在运行SELECT子查询后才会发生。