mybatis实体类的关联查询

MyBatis 可以通过关联查询来获取实体类之间的关系。关联查询主要用于处理一对一、一对多和多对一的关系。以下是一些示例，展示了如何在 MyBatis 中执行关联查询。

1. 一对一关系：

假设我们有两个实体类：User 和 UserDetails，其中 User 与 UserDetails 之间存在一对一的关系。

首先，创建 User 和 UserDetails 的实体类：

public class User {     private int id;     private String name;     private UserDetails userDetails;     // getter and setter methods }  public class UserDetails {     private int id;     private String address;     private int userId;     // getter and setter methods } 

然后，在 UserMapper.xml 文件中编写关联查询：

    <id property="id" column="user_id"/>    <result property="name" column="user_name"/>    <association property="userDetails" javaType="UserDetails" resultMap="UserDetailsResultMap"/> </resultMap><resultMap id="UserDetailsResultMap" type="UserDetails">     <id property="id" column="user_details_id"/>    <result property="address" column="address"/>    <result property="userId" column="user_id"/> </resultMap><select id="getUserWithDetails" resultMap="UserResultMap">     SELECT u.id as user_id, u.name as user_name, ud.id as user_details_id, ud.address, ud.user_id     FROM user u     JOIN user_details ud ON u.id = ud.user_id     WHERE u.id = #{userId} </select> 

2. 一对多关系：

假设我们有两个实体类：Author 和 Book，其中 Author 与 Book 之间存在一对多的关系。

首先，创建 Author 和 Book 的实体类：

public class Author {     private int id;     private String name;     private List<Book> books;     // getter and setter methods }  public class Book {     private int id;     private String title;     private int authorId;     // getter and setter methods } 

然后，在 AuthorMapper.xml 文件中编写关联查询：

    <id property="id" column="author_id"/>    <result property="name" column="author_name"/>    <collection property="books" ofType="Book" resultMap="BookResultMap"/> </resultMap><resultMap id="BookResultMap" type="Book">     <id property="id" column="book_id"/>    <result property="title" column="title"/>    <result property="authorId" column="author_id"/> </resultMap><select id="getAuthorWithBooks" resultMap="AuthorResultMap">     SELECT a.id as author_id, a.name as author_name, b.id as book_id, b.title, b.author_id     FROM author a     LEFT JOIN book b ON a.id = b.author_id     WHERE a.id = #{authorId} </select> 

3. 多对一关系：

假设我们有两个实体类：Student 和 Classroom，其中 Student 与 Classroom 之间存在多对一的关系。

首先，创建 Student 和 Classroom 的实体类：

public class Student {     private int id;     private String name;     private Classroom classroom;     // getter and setter methods }  public class Classroom {     private int id;     private String name;     // getter and setter methods } 

然后，在 StudentMapper.xml 文件中编写关联查询：

    <id property="id" column="student_id"/>    <result property="name" column="student_name"/>    <association property="classroom" javaType="Classroom" resultMap="ClassroomResultMap"/> </resultMap><resultMap id="ClassroomResultMap" type="Classroom">     <id property="id" column="classroom_id"/>    <result property="name" column="classroom_name"/> </resultMap><select id="getStudentWithClassroom" resultMap="StudentResultMap">     SELECT s.id as student_id, s.name as student_name, c.id as classroom_id, c.name as classroom_name     FROM student s     JOIN classroom c ON s.classroom_id = c.id     WHERE s.id = #{studentId} </select> 

这些示例展示了如何在 MyBatis 中执行一对一、一对多和多对一的关联查询。你可以根据自己的需求调整这些示例以适应你的项目。