Question

我有一个表单和一个插入按钮，当我点击按钮时 - 字段会进入这些表格（我没有把所有字段放在这里，因为它们对我的问题不重要）。

表格：

CREATE TABLE SafetyAct (
SafetyAct_id int identity(1,1),
Username varchar(50),
SafetyType_id int,
constraint pk_SafetyAct_id 
           primary key (SafetyAct_id),
constraint fk_Users_SafetyAct 
           foreign key(Username) 
           references Users(Username)
           on delete cascade
)
CREATE TABLE Product (
Product_id int identity(1,1) primary key,
SafetyAct_id int,
Cause_id int,
constraint fk_SafetyAct_Product
           foreign key(SafetyAct_id)
           references SafetyAct(SafetyAct_id)
           on delete cascade,
constraint fk_Cause_Product
           foreign key(Cause_id)
           references Cause(Cause_id) 
           on delete cascade
)
CREATE TABLE SafetyIntervention (
SafetyIntervention_id int identity(1,1) primary key,
SafetyAct_id int,
Cause_id int,
constraint fk_SafetyAct_SafetyIntervention 
           foreign key(SafetyAct_id)
           references SafetyAct(SafetyAct_id)
           on delete cascade,
constraint fk_Cause_SafetyIntervention
           foreign key(Cause_id)
           references Cause(Cause_id)
           on delete cascade
)
CREATE TABLE Cause (
Cause_id int primary key,
Cause_name varchar(80)
)

我想编写一个显示字段的查询 - SafetyAct_id和Cause_name。在Cause_name字段中我有一个问题，因为我希望查询将显示原因名称drom来自SafetyIntervension表的产品表或（当然要将它连接到Cause表，因为我只有这些表中的cause_id - foriegn键）并且我不知道如何在同一查询中编写INNER JOIN和OR。

我是新手，所以请耐心等待。

谢谢！

Answer 1

SELECT  SA.SafetyAct_id,
        C.Cause_name
FROM SafetyAct SA
LEFT JOIN Product P
    ON SA.SafetyAct_id = P.SafetyAct_id
LEFT JOIN SafetyIntervention SI
    ON SA.SafetyAct_id = SI.SafetyAct_id
LEFT JOIN Cause C
    ON ISNULL(P.Cause_id,SI.Cause_id) = C.Cause_id

Answer 2

or很简单。所以，基于Lamak的代码：

SELECT SA.SafetyAct_id,
       C.Cause_name
FROM SafetyAct SA LEFT JOIN
     Product P
     ON SA.SafetyAct_id = P.SafetyAct_id LEFT JOIN
     SafetyIntervention SI
     ON SA.SafetyAct_id = SI.SafetyAct_id LEFT JOIN
     Cause C
     ON C.Cause_id = P.Cause_id OR C.Cause_Id = SI.Cause_id;
--------------------------------^

您可以在OR条件中使用ON。

但是，当join条件包含on条件或用于连接的列上的函数时，SQL引擎（包括SQL Server）优化or通常具有挑战性。因此，以下内容通常更有效：

SELECT SA.SafetyAct_id,
       COALESCE(Cp.Cause_name, Csi.Cause_Name) as Cause_Name
FROM SafetyAct SA LEFT JOIN
     Product P
     ON SA.SafetyAct_id = P.SafetyAct_id LEFT JOIN
     SafetyIntervention SI
     ON SA.SafetyAct_id = SI.SafetyAct_id LEFT JOIN
     Cause Cp
     ON Cp.Cause_id = P.Cause_id LEFT JOIN
     Cause Csi
     ON Csi.Cause_Id = SI.Cause_id;

如果只有部分记录有两种类型的原因，请添加：

WHERE Cp.Cause_Id IS NOT NULL OR Csi.Cause_Id IS NOT NULL;

使用INNER JOIN AND OR编写查询

2 个答案: