“按照规则分配外键” 在数据库设计中,我们常常需要在表之间建立关联关系,其中外键是实现关联关系的重要手段之一。在分配外键时,可以按照一定的规则进行分配,比如说按照时间顺序、按照字母顺序、按照数值大小等等。这样可以使得关联关系更加规范化和稳定。 具体
1.在 Departments 表中,添加部门编号字段和部门名称字段: CREATE TABLE Departments ( DepartmentId int PRIMARY KEY, DepartmentName varchar(100) );
2.在 Employees 表中,添加外键字段 DepartmentId: CREATE TABLE Employees ( EmployeeId int PRIMARY KEY, EmployeeName varchar(100), DepartmentId int FOREIGN KEY REFERENCES Departments(DepartmentId) );
3.在插入数据时,自动为外键字段 DepartmentId 赋值,按照部门名称的字母顺序为其赋值: INSERT INTO Departments(DepartmentId, DepartmentName) VALUES (1, 'Marketing');
INSERT INTO Departments(DepartmentId, DepartmentName) VALUES (2, 'Sales');
INSERT INTO Departments(DepartmentId, DepartmentName) VALUES (3, 'IT');
INSERT INTO Employees(EmployeeId, EmployeeName, DepartmentId) VALUES (1, 'John Doe', (SELECT DepartmentId FROM Departments ORDER BY DepartmentName LIMIT 1));
INSERT INTO Employees(EmployeeId, EmployeeName, DepartmentId) VALUES (2, 'Jane Doe', (SELECT DepartmentId FROM Departments ORDER BY DepartmentName LIMIT 1));
INSERT INTO Employees(EmployeeId, EmployeeName, DepartmentId) VALUES (3, 'Bob Smith', (SELECT DepartmentId FROM Departments ORDER BY DepartmentName LIMIT 1));
这个示例演示了如何按照规则为外键字段赋值,这里使用了子查询和 LIMIT 子句来获取部门信息表中名称排名第一的部门编号。在实际应用中,可能需要根据具体情况进行修改,比如说根据时间顺序来为外键赋值。