Question

让我们说，银行架构如下：

Employee = {*ID*,salary}
Branch = {*BranchNum*, Location}

如果关系同时具有参与约束和关键约束：

Employee ==> workAt -- Branch  //every employee must work at one branch

我可以通过将 BranchNum 设置为 NOT NULL 来强制参与约束：

CREATE TABLE employee (
    ID           INTEGER,
    BranchNum    INTEGER     NOT NULL, 
    PRIMARY KEY (ID)
    FOREIGN KEY (ID) REFERENCES Employee
    FOREIGN KEY (BranchNum) REFERENCES Branch
)

然而，如果只有参与约束但没有关键约束：

Employee === workAt -- Branch  //every employee must work at one or some branches

创建表格时如何设置参与约束？

Answer 1

如果你想：

//每位员工必须在一个或一些分支机构工作

然后你有一个1-n的关系。您可以使用联结表实现此目的：

CREATE TABLE EmployeeBranches (
    EmployeeBranchId INTEGER AUTO_INCREMENT PRIMARY KEY,
    EmployeeID INTEGER NOT NULL,
    BranchNum INTEGER NOT NULL,
    FOREIGN KEY (EmployeeID) REFERENCES Employee(EmployeeId)
    FOREIGN KEY (BranchNum) REFERENCES Branch(BranchNum)
);

SQL：如何在表中创建参与约束

1 个答案: