Mybatis关联查询 一对多和多对一

• 一对多的第一种写法

首先是有两张表(学生表Student和老师Teacher表)，为了更易懂，这里只设置了最简单的几个必要字段。表结构如下图

Student表：

Teacher表：

• 创建实体bean

Teacher.java:

public class Teacher {
 
	private Integer id;
	private String name;
	private String className;
	private List<Student> students;
 
	public List<Student> getStudents() {
		return students;
	}
 
	public void setStudents(List<Student> students) {
		this.students = students;
	}
 
	public Integer getId() {
		return id;
	}
 
	public void setId(Integer id) {
		this.id = id;
	}
 
	public String getName() {
		return name;
	}
 
	public void setName(String name) {
		this.name = name;
	}
 
	public String getClassName() {
		return className;
	}
 
	public void setClassName(String className) {
		this.className = className;
	}
 
}

Sfudent.java

public class Student {
 
	private Integer id;
	private String name;
	private Integer teacherId;
	private String className;
	private Teacher teacher;
	
 
	public Teacher getTeacher() {
		return teacher;
	}
 
	public void setTeacher(Teacher teacher) {
		this.teacher = teacher;
	}
 
	public Integer getId() {
		return id;
	}
 
	public void setId(Integer id) {
		this.id = id;
	}
 
	public String getName() {
		return name;
	}
 
	public void setName(String name) {
		this.name = name;
	}
 
	public Integer getTeacherId() {
		return teacherId;
	}
 
	public void setTeacherId(Integer teacherId) {
		this.teacherId = teacherId;
	}
 
	public String getClassName() {
		return className;
	}
 
	public void setClassName(String className) {
		this.className = className;
	} 

} 

• 下面重点来了：配置Mapper.xml文件：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.tz.mybatis.dao.studentDao">  
	
	<!-- //一对多的第一种写法，一般考虑到性能问题，不会这么实现// -->
	<resultMap type="Teacher" id="teacherMap">
		<id column="id" property="id"/>
		<result column="name" property="name"/>
		<collection property="students" ofType="Student" column="id"> <!-- 这里要注意的是column对应的是teacher中的主键id，而且需是表字段名，传给student的外键teacherId -->
			<id column="sid" property="id"/><!-- 这里的column对应的是下面查询的别名，而不是表字段名 -->
			<result column="sname" property="name"/><!-- property对应JavaBean中的属性名 -->
			<result column="className" property="className"/>
		</collection>
	</resultMap>
	
	
	<!-- 查询所有的老师级各自的所有学生 -->
	<select id="getTeachers" parameterType="Teacher" resultMap="teacherMap">
		SELECT
			t.id,
			t.NAME,
			t.class_Name,
			s.id AS sid,
			s. NAME AS sname,
			s.class_name as className
		FROM
			teacher t
		LEFT JOIN student s ON t.id = s.teacher_id
	</select>
</mapper>

• 测试类：

•  
  public class TeacherTest {
   
  	private SqlSessionFactory sqlSessionFactory;
  	
  	@Before
  	public void init() throws IOException {
  		String resource = "mybatis-config.xml";
  		InputStream inputStream = Resources.getResourceAsStream(resource);
  		sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
  	}
  	
  	@Test
  	public void getTeachers() {
  		SqlSession session = sqlSessionFactory.openSession();
  		List<Teacher> list = session.selectList("com.tz.mybatis.dao.studentDao.getTeachers");
  		System.out.println(list);
  	}
  	
  }
  

   

• 一对多的第二种写法

<!-- //一对多的第二种写法/ -->
	<resultMap type="Teacher" id="teacherMaps">
		<id column="id" property="id"/>
		<result column="name" property="name"/>
		<result column="class_name" property="className"/>
		<collection property="students" ofType="Student" select="getStudents" column="id">
<!-- 这里要注意的是column对应的是student中的外键，而且需是表字段名 -->
		</collection>
	</resultMap>
	
	
	<!-- 查询所有的老师级各自的所有学生 -->
	<select id="getAllTeacher" parameterType="Teacher" resultMap="teacherMaps">
		SELECT
			t.id,
			t.NAME,
			t.class_name
		FROM
			teacher t
	</select>
	
	<select id="getStudents" parameterType="int" resultType="Student">
		select 
			s.id,
			s. NAME,
			s.class_name as className
		from student s
		where teacher_id = #{id} <!-- 这里要注意的是#{id}对应的id是student中的外键，而且需是表字段名 -->
	</select>

测试类：

@Test
	public void getTeachers2() {
		SqlSession session = sqlSessionFactory.openSession();
		List<Teacher> list = session.selectList("com.tz.mybatis.dao.studentDao.getAllTeacher");
		System.out.println(list);
	}

• 多对一的写法 查询学生信息(多对一)：

首先还是配置文件：

<resultMap type="Student" id="studentMap">
		<id column="id" property="id"/>
		<result column="name" property="name"/>
		<result column="class_name" property="className"/>
		<result column="teacher_id" property="teacherId"/>
		<association property="teacher" select="getTeacher" column="teacher_id" javaType="Teacher">
		<!-- 这里要注意的是column对应的是student中的外键，而且需是表字段名 -->
		</association>
	</resultMap>
	
	
	<select id="getStudent" resultMap="studentMap">
		SELECT
			s.id,
			s.name,
			s.class_name,
			s.teacher_id
		FROM
			student s
	</select>
	
	<select id="getTeacher" resultType="Teacher" parameterType="int">
		SELECT
			t.id,
			t.name,
			t.class_name as className 
		FROM teacher t 
		where id = #{teacher_id}
	</select>

测试类：

@Test
	public void getStudents() {
		SqlSession session = sqlSessionFactory.openSession();
		List<Student> list = session.selectList("com.tz.mybatis.dao.studentDao.getStudent");
		System.out.println(list);
	}

 最后：当然如果不想配置这么麻烦的信息，可以直接写一个关联查询的SQL语句，返回结果直接由Map接受即可。不过这样就不太符合面向对象的理念了。

最后

以上就是爱笑心锁为你收集整理的Mybatis关联查询 一对多和多对一的全部内容，希望文章能够帮你解决Mybatis关联查询 一对多和多对一所遇到的程序开发问题。

如果觉得靠谱客网站的内容还不错，欢迎将靠谱客网站推荐给程序员好友。