Mysql 选择最特定的联接结果

我试图让mysql返回一个shopitem最具体的折扣，因为可能会有几个折扣。我的表格和数据如下（不相关的列被省略）：

我预期的查询输出是

itemId  price  discountPrice
===================================
Item1   100    80
Item2   100    60
Item3   100    40
Item4   100    (null)

正如你所看到的，我的规则是

供应商折扣是最不明确的

供应商+品牌折扣更具体

ItemId折扣是最具体的

但是，带有或在子句上的正常左连接将返回所有组合，而不是最具体的折扣。我怎样才能做到这一点

---

选择item.itemId、item.price、discount.discountPrice从项目左侧加入折扣（item.itemId=discount.itemId）或（item.brand=discount.brand和item.supplier=discount.supplier）或（item.supplier=discount.supplier和discount.brand为NULL）

我会这样做： 对所有三个折扣使用单独的左连接，并从中选择最具体的折扣

Select 
    i.itemId, 
    i.price,
    coalesce(spec3.discountPrice, spec2.discountPrice, spec1.discountPrice)
from item i
left join Discount spec3 on (i.itemId = spec3.itemId)
left join Discount spec2 on (i.supplier = spec2.supplier and i.brand = spec2.brand)
left join Discount spec1 on (i.supplier = spec1.supplier)

---

上面的查询可能包含一些语法错误，我附近没有mysql服务器来实际运行它。

下面是我的方法： 对所有三个折扣使用单独的左连接，并从中选择最具体的折扣

Select 
    i.itemId, 
    i.price,
    coalesce(spec3.discountPrice, spec2.discountPrice, spec1.discountPrice)
from item i
left join Discount spec3 on (i.itemId = spec3.itemId)
left join Discount spec2 on (i.supplier = spec2.supplier and i.brand = spec2.brand)
left join Discount spec1 on (i.supplier = spec1.supplier)

上面的查询可能包含一些语法错误，我附近没有mysql服务器来实际运行它。

查询：

SELECT i.itemId, 
       i.price, 
       COALESCE(d.discountPrice, d2.discountPrice, d3.discountPrice) AS discountPrice 
FROM item i
LEFT JOIN discount d 
  ON i.itemId = d.itemId
LEFT JOIN discount d2
  ON i.brand = d2.brand
  AND i.supplier = d2.supplier 
LEFT JOIN discount d3
 ON i.supplier = d3.supplier 
 AND d3.brand IS NULL

结果:

| ITEMID | PRICE | DISCOUNTPRICE |
----------------------------------
|  Item1 |   100 |            80 |
|  Item2 |   100 |            60 |
|  Item3 |   100 |            40 |
|  Item4 |   100 |        (null) |

查询：

SELECT i.itemId, 
       i.price, 
       COALESCE(d.discountPrice, d2.discountPrice, d3.discountPrice) AS discountPrice 
FROM item i
LEFT JOIN discount d 
  ON i.itemId = d.itemId
LEFT JOIN discount d2
  ON i.brand = d2.brand
  AND i.supplier = d2.supplier 
LEFT JOIN discount d3
 ON i.supplier = d3.supplier 
 AND d3.brand IS NULL

结果:

| ITEMID | PRICE | DISCOUNTPRICE |
----------------------------------
|  Item1 |   100 |            80 |
|  Item2 |   100 |            60 |
|  Item3 |   100 |            40 |
|  Item4 |   100 |        (null) |