SQL Server-查询数据库以查找两列之间的差异

我试图显示不同蔬菜的每个主方向（N、E、S、W）的水分测量值之间的差异。我想我的思路是对的，但不确定

下面是CREATETABLE语句

CREATE TABLE Location
(
     LocationID int IDENTITY(0, 1) NOT NULL
,    LocationExposure varchar(50) NOT NULL
,    LocationSunlightPercentage numeric(3,2) NOT NULL
,    LocationMoistureLevel numeric(3, 2) NOT NULL
,    CONSTRAINT PK_Location PRIMARY KEY (LocationID)
)

CREATE TABLE Crop
(
     CropID int IDENTITY(0, 1) NOT NULL
,    CropName varchar(50)
,    CropOptimalSunlightPercentage numeric(3,2) NOT NULL
,    CropOptimalMoistureLevel numeric(3,2) NOT NULL
,    CropExpectedHarvestWeight numeric(3,2) NOT NULL
,    CONSTRAINT PK_Crop PRIMARY KEY (CropID)
)

CREATE TABLE Sowing
(
     CropID int NOT NULL
,    TechnicianID int NOT NULL
,    LocationID int NOT NULL
,    SowingDate date NOT NULL
,    SowingSeedCount int NOT NULL
,    CONSTRAINT PK_Sowing PRIMARY KEY (CropID, TechnicianID, LocationID)
,    CONSTRAINT FK_SowingCrop FOREIGN KEY (CropID) REFERENCES Crop(CropID) ON DELETE CASCADE
,    CONSTRAINT FK_SowingTechnician FOREIGN KEY (TechnicianID) REFERENCES Technician(TechnicianID) ON DELETE CASCADE
,    CONSTRAINT FK_SowingLocation FOREIGN KEY (LocationID) REFERENCES Location(LocationID) ON DELETE CASCADE
)

以下是虚拟数据

INSERT INTO Location VALUES ('East', 0.28, 0.80)
INSERT INTO Location VALUES ('North', 0.17, 0.84)
INSERT INTO Location VALUES ('West', 0.38, 0.48)
INSERT INTO Location VALUES ('South', 0.45, 0.66)

INSERT INTO Crop VALUES ('Carrot', 0.26, 0.82, 0.08)
INSERT INTO Crop VALUES ('Beet', 0.44, 0.80, 0.04)
INSERT INTO Crop VALUES ('Corn', 0.44, 0.76, 0.26)
INSERT INTO Crop VALUES ('Tomato', 0.42, 0.80, 0.16)
INSERT INTO Crop VALUES ('Radish', 0.28, 0.84, 0.02)

INSERT INTO Sowing VALUES (0, 0, 0, '20050418', 28)
INSERT INTO Sowing VALUES (0, 1, 1, '20050414', 14)
INSERT INTO Sowing VALUES (1, 0, 2, '20050418', 36)
INSERT INTO Sowing VALUES (2, 1, 3, '20050414', 20)
INSERT INTO Sowing VALUES (2, 2, 2, '20050419', 12)
INSERT INTO Sowing VALUES (3, 3, 3, '20050425', 38)
INSERT INTO Sowing VALUES (4, 2, 0, '20050430', 30)

到目前为止

SELECT CropName AS [Crop], LocationExposure AS [Location], CropOptimalMoistureLevel AS [Water Needed],  
       LocationMoistureLevel AS [Water Available], 
       CropOptimalMoistureLevel - LocationMoistureLevel AS [Difference]
FROM Sowing S JOIN
     Location L
     ON L.LocationID = S.LocationID JOIN
     Crop C
     ON C.CropID = S.CropID

---

您不需要

表。您只需要一个
交叉连接
：

SELECT c.CropName AS [Crop], l.LocationExposure AS [Location],
       c.CropOptimalMoistureLevel AS [Water Needed],  
       l.LocationMoistureLevel AS [Water Available], 
       (c.CropOptimalMoistureLevel - l.LocationMoistureLevel) AS [Difference]
FROM Location L CROSS JOIN
     Crop C
ORDER BY C.CropID, l.LocationExposure;

你的查询有什么问题？@GordonLinoff交叉连接不好吗？笛卡尔连接不好吗？@User123。只有在没有正确使用的情况下。这似乎是你想要的。只有当它们建立了你不需要或不想要的组合时，它们才是不好的。但在这种情况下，您需要知道每种可能的作物和位置组合的结果。交叉连接是合乎逻辑的选择！