如何为此编写sql？

Question

我有这样的表

Customer
-ID
-name
-address

Business
-ID
-name
-type

Discount
-ID
-amount
-BusinessID
-position

UsedDiscounts
-CustomerID
-DiscountID

企业有很多折扣。

用户使用过很多折扣，记录在 UsedDiscounts 中。

用户只能按顺序使用折扣，由位置定义。折扣 1，然后是折扣 2。因此，即使企业有 10 个折扣，客户有资格获得的一个是该企业使用折扣的位置 + 1。

目标：获得用户有资格享受的所有折扣。

我的方法是对折扣和二手折扣进行左排除联接。

因此，获得所有折扣减去使用过的折扣，然后以某种方式最小化位置并获得所有“合格”的折扣。但是，我可能可以在 SQL 中实现这一点，但我不知道如何......

示例不完整的 SQL 如下所示

SELECT *, min(gd.position)  FROM

(SELECT * FROM "Deals" as d WHERE (d.active = true) AND (d.latitude BETWEEN 40 AND 41) AND (d.longitude BETWEEN -75 AND -70)) AS gd

LEFT JOIN

(SELECT du."DealId" FROM "DealsUsed" AS du WHERE du."CustomerId" = 1) AS bd

ON gd.id = bd."DealId"
WHERE bd."DealId" IS NULL
GROUP BY gd."UserId";

输出错误

Sample data:

Customer
--------
id   name  address
0    Tobby   93903903
1    Emi     3839039
2    Loop    393030

Business
--------
id   name   type
0    Cool   flower
1    Corner car
2    New    deli
3    Side   printing
4    Big    car

Discount
--------
id  amount  businessId  position
0   10       0              0
1   22       3              1
2   10       3              2
3   43       2              0
4   23       5              0
5   10       5              1

Used Discount
----------
customerId    discountId
1              2


outcome for customer 1 , emi, shouuld be 
Discounts
--------
id  amount  businessId  position
0   10       0              0
4   23       5              0
3   43       2              0
5   10       5              1

Answer 1

您的左连接不会生效，因为您在第二个查询中使用了 where 子句：

删除 where 子句：

SELECT *, min(gd.position)  FROM

(SELECT * FROM "Deals" as d WHERE (d.active = true) AND (d.latitude BETWEEN 40 AND 41) AND (d.longitude BETWEEN -75 AND -70)) AS gd

LEFT JOIN

(SELECT du."DealId" FROM "DealsUsed" AS du) AS bd

ON gd.id = bd."DealId"
WHERE bd."DealId" IS NULL
GROUP BY gd."UserId";

不建议将Group by 与Select * 一起使用。只需选择您需要的字段。比如：

SELECT gd."UserId", min(gd.position)  FROM

(SELECT * FROM "Deals" as d WHERE (d.active = true) AND (d.latitude BETWEEN 40 AND 41) AND (d.longitude BETWEEN -75 AND -70)) AS gd

LEFT JOIN

(SELECT du."DealId" FROM "DealsUsed" AS du) AS bd

ON gd.id = bd."DealId"
WHERE bd."DealId" IS NULL
GROUP BY gd."UserId";

你可以在 on like 之后使用 where 子句：

SELECT gd."UserId",bd."CustomerId", min(gd.position)  FROM

(SELECT * FROM "Deals" as d WHERE (d.active = true) AND (d.latitude BETWEEN 40 AND 41) AND (d.longitude BETWEEN -75 AND -70)) AS gd

LEFT JOIN

(SELECT du."DealId" FROM "DealsUsed" AS du) AS bd

ON gd.id = bd."DealId"
WHERE bd."DealId" IS NULL and bd."CustomerId" = 1
GROUP BY gd."UserId";