Question

让我们简单一点：

USE Example1

    CREATE TABLE Person
    (PersonID int PRIMARY KEY IDENTITY(1,1),
    FirstName nchar(20) NOT NULL,
    LastName  nchar(20) NOT NULL,
    Salary money NOT NULL
    ) 

        CREATE TABLE Student
        (StudentID int PRIMARY KEY IDENTITY(1,1),
        FirstName nchar(20) NOT NULL,
        LastName  nchar(20) NOT NULL,
        FatherID int NOT NULL,
        MotherID int NOT NULL,
        CONSTRAINT fk_Student_FatherID FOREIGN KEY (FatherID)
        REFERENCES Person(PersonID),
        CONSTRAINT fk_Student_MotherID FOREIGN KEY (MotherID)
        REFERENCES Person(PersonID)
        ) 

    CREATE TABLE Registration
    (RegistrationID int PRIMARY KEY IDENTITY(1,1),
    StudentID int NOT NULL,
    Date datetime NOT NULL,
    MonthlyPayment ??????????
    CONSTRAINT fk_Registration_StudentID FOREIGN KEY (StudentID)
    REFERENCES Student(StudentID)
    ) 

    INSERT INTO Person VALUES ('John','Doe','1000')
    INSERT INTO Person VALUES ('Mary','Poppins','800')

    INSERT INTO Student VALUES ('Gary','Doe', 1, 2)

    INSERT INTO Registration VALUES (1, getdate(),???)

我有一个学生要在学校进行注册并且每月付款FatherSalary*0.5 + MotherSalary*0.5，但我不知道如何实现这一点。我是SQL的新手，也许这很简单，我应该知道如何制作它，但我没有，我需要帮助。

Answer 1

您确定表格中需要MonthlyPayment列吗？

您可以创建没有Registration字段的表MonthlyPayment，然后创建一个视图

create view vw_Registration
as
    select
        R.RegistrationID,
        R.StudentID,
        R.Date,
        F.Salary * 0.5 + M.Salary * 0.5 as MonthlyPayment
    from Registration as R
        left outer join Student as S on S.StudentID = R.StudentID
        left outer join Person as F on F.PersonID = S.FatherID
        left outer join Person as M on M.PersonID = S.MotherId

SQL FIDDLE EXAMPLE

Answer 2

如果表达式“FatherSalary * 0.5 + MotherSalary * 0.5”不会改变，那么您可以使用触发器假设。您的触发器将检查您的注册表中的插入。在插入时，它将获得学生ID，然后使用此ID，它将从您的学生表和您的Person表中获取必要的数据。在那一刻，您将可以访问父亲和母亲的Money列。计算结果，并为您插入触发器。

Answer 3

一个视图听起来不错，但我可以想象一个场景，你想根据某个时间点的可用数据计算每月付款，这样每当父亲或母亲有一个月付款时，每月付款就不会改变工资变动......

在这种情况下，你可以使用它：

INSERT INTO Registration 
(StudentID, Date, MonthlyPayment)
SELECT S.StudentID, getdate(), ISNULL(F.Salary, 0) * 0.5 + ISNULL(M.Salary, 0) * 0.5
FROM Student as S
left outer join Person as F on F.PersonID = S.FatherID
left outer join Person as M on M.PersonID = S.MotherId
WHERE S.StudentID = 1

SQL Fiddle

使用其他值计算值

3 个答案: