Java MyBatis如何映射关系数据

MyBatis是一个优秀的持久层框架，它支持定制化SQL、存储过程以及高级映射。在MyBatis中，可以通过配置映射文件（mapper.xml）来实现关系数据的映射。以下是一些常见的关系数据映射方法：

1. 一对一关系映射：

假设有两个表：user 和 user_profile，它们之间是一对一的关系。首先，创建两个对应的实体类 User 和 UserProfile。

User.java:

public class User {
    private Integer id;
    private String username;
    private UserProfile userProfile;
    // getter and setter methods
}

UserProfile.java:

public class UserProfile {
    private Integer id;
    private String address;
    private User user;
    // getter and setter methods
}

在 UserMapper.xml 中配置一对一关系映射：

<mapper namespace="com.example.mapper.UserMapper">
    <resultMap id="UserResultMap" type="User">
        <id property="id" column="id"/>
        <result property="username" column="username"/>
        <association property="userProfile" javaType="UserProfile">
            <id property="id" column="profile_id"/>
            <result property="address" column="address"/>
        </association>
    </resultMap>

    <select id="getUserById" resultMap="UserResultMap">
        SELECT u.id, u.username, p.id as profile_id, p.address
        FROM user u
        LEFT JOIN user_profile p ON u.id = p.user_id
        WHERE u.id = #{id}
    </select>
</mapper>

2. 一对多关系映射：

假设有两个表：department 和 employee，它们之间是一对多的关系。首先，创建两个对应的实体类 Department 和 Employee。

Department.java:

public class Department {
    private Integer id;
    private String name;
    private List<Employee> employees;
    // getter and setter methods
}

Employee.java:

public class Employee {
    private Integer id;
    private String name;
    private Integer departmentId;
    private Department department;
    // getter and setter methods
}

在 DepartmentMapper.xml 中配置一对多关系映射：

<mapper namespace="com.example.mapper.DepartmentMapper">
    <resultMap id="DepartmentResultMap" type="Department">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <collection property="employees" ofType="Employee">
            <id property="id" column="employee_id"/>
            <result property="name" column="name"/>
            <result property="departmentId" column="department_id"/>
        </collection>
    </resultMap>

    <select id="getDepartmentById" resultMap="DepartmentResultMap">
        SELECT d.id, d.name, e.id as employee_id, e.name, e.department_id
        FROM department d
        LEFT JOIN employee e ON d.id = e.department_id
        WHERE d.id = #{id}
    </select>
</mapper>

3. 多对多关系映射：

假设有两个表：student 和 course，它们之间是多对多的关系。首先，创建两个对应的实体类 Student 和 Course，以及一个中间表 student_course。

Student.java:

public class Student {
    private Integer id;
    private String name;
    private List<Course> courses;
    // getter and setter methods
}

Course.java:

public class Course {
    private Integer id;
    private String name;
    private List<Student> students;
    // getter and setter methods
}

在 StudentMapper.xml 中配置多对多关系映射：

<mapper namespace="com.example.mapper.StudentMapper">
    <resultMap id="StudentResultMap" type="Student">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <collection property="courses" ofType="Course">
            <id property="id" column="course_id"/>
            <result property="name" column="name"/>
        </collection>
    </resultMap>

    <select id="getStudentById" resultMap="StudentResultMap">
        SELECT s.id, s.name, c.id as course_id, c.name
        FROM student s
        LEFT JOIN student_course sc ON s.id = sc.student_id
        LEFT JOIN course c ON s.id = c.id
        WHERE s.id = #{id}
    </select>
</mapper>

通过以上示例，可以在 MyBatis 中实现关系数据的映射。注意根据实际情况调整 SQL 查询语句和 resultMap 配置。