我正在尝试让 mysql 返回商店商品的最具体折扣,因为可能有多个折扣适用。我的表格和数据如下(不相关的列已被省略):
Item
itemId brand supplier price
====== ===== ======== =====
Item1 Brand1 Supply1 100
Item2 Brand2 Supply1 100
Item3 Brand3 Supply1 100
Item4 Brand4 Supply2 100
Discount
discountId itemId brand supplier discountPrice
========== ====== ====== ======== =============
Discount1 (null) (null) Supply1 80
Discount2 (null) Brand2 Supply1 60
Discount3 Item3 (null) (null) 40
我期望的查询输出是
itemId price discountPrice
===================================
Item1 100 80
Item2 100 60
Item3 100 40
Item4 100 (null)
如你所见,我的规则是
- 供应商折扣最不明确
- 供应商+品牌折扣更具体
- ItemId 折扣最具体
然而,在子句上使用 or 进行正常的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 from item left join discount on (item.itemId = discount.itemId) or (item.brand = discount.brand and item.supplier = discount.supplier ) 或 (item.supplier = discount.supplier AND discount.brand IS NULL)
最佳答案
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) |
关于mysql - 选择最具体的连接结果,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/17588538/