1、环境搭建
1.1、数据库中创建Teacher表和Student 表，并插入内容
CREATE TABLE `teacher`(
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
PRIMARY KEY(`id`)
) ENGINE = INNODB DEFAULT CHARSET=utf8
INSERT INTO teacher(`id`,`name`) VALUES('1','秦老师');
CREATE TABLE `student`(
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
`tid` INT(10) DEFAULT NULL,
PRIMARY KEY(`id`),
KEY `fktid` (`tid`),
CONSTRAINT `fktid` FOREIGN KEY (`tid`)REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8
INSERT INTO `student` (`id`,`name`,`tid`) VALUES('1','小明','1');
INSERT INTO `student` (`id`,`name`,`tid`) VALUES('2','小红','1');
INSERT INTO `student` (`id`,`name`,`tid`) VALUES('3','小张','1');
INSERT INTO `student` (`id`,`name`,`tid`) VALUES('4','小李','1');
INSERT INTO `student` (`id`,`name`,`tid`) VALUES('5','小王','1');
1,2、创建实体类Teacher、Student
public class Student {
private int id;
private String name;
//学生需要关联老师
private Teacher teacher;
public Student() {
}
public Student(int id, String name, Teacher teacher) {
this.id = id;
this.name = name;
this.teacher = teacher;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
@Override
public String toString() {
return "Student{" +
"id=" + id +
", name='" + name + '\'' +
", teacher=" + teacher +
'}';
}
}
public class Teacher {
private int id;
private String name;
public Teacher() {
}
public Teacher(int id, String name) {
this.id = id;
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Teacher{" +
"id=" + id +
", name='" + name + '\'' +
'}';
}
}
1.3、建立Mapper接口
public interface StudentMapper {
}
public interface TeacherMapper { //里面的内容是用于测试的
@Select("select * from teacher where id =#{tid}")
Teacher getTeacher(@Param("tid") int id);
}
1.4、建立Mapper.xml文件 （创建空文件，然后把mybatis.xml中的内容复制过去，删掉多余内容，修改两个地方就行）
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.kuang.dao.StudentMapper">
</mapper>
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.kuang.dao.TeacherMapper">
</mapper>
1.5、在核心配置文件中绑定注册我们的Mapper接口或者文件
<mappers>
<mapper class="com.kuang.dao.TeacherMapper"/>
<mapper class="com.kuang.dao.StudentMapper"/>
</mappers>
1.6、测试查询是否能够成功
public class MyTest {
public static void main(String[] args) {
SqlSession sqlSession = MybatisUtils.getSqlSession();
TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
Teacher teacher = mapper.getTeacher(1);
System.out.println(teacher);
sqlSession.close();
}
}
2、多对1的处理
2.1按照查询嵌套处理
<!--
思路
1、查询所有的学生信息
2、根据查询出来的学生的tid寻找对应的老师
-->
<select id="getStudent" resultMap="StudentTeacher">
select * from student;
</select>
<resultMap id="StudentTeacher" type="Student">
<result property="id" column="id"/>
<result property="name" column="name"/>
<!--复杂的属性需要单独处理
对象：association
集合：collection
-->
<association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
</resultMap>
<select id="getTeacher" resultType="Teacher">
select * from teacher where id = #{id}
</select>
2.2按照结果嵌套查询
<!--按照结果嵌套处理-->
<select id="getStudent2" resultMap="StudentTeacher2">
select s.id sid,s.name sname,t.name tname from student s,teacher t where s.tid = t.id;
</select>
<resultMap id="StudentTeacher2" type="Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<association property="teacher" javaType="Teacher">
<result property="name" column="tname"/>
</association>
</resultMap>
3、一对多
1、按照结果嵌套处理
<!--按结果嵌套查询-->
<select id="getTeacher" resultMap="TeacherStudent">
select s.id sid, s.name sname,t.name tname,t.id tid
from student s,teacher t
where s.tid = t.id and t.id = #{tid}
</select>
<resultMap id="TeacherStudent" type="Teacher">
<result property="id" column="tid"/>
<result property="name" column="tname"/>
<!--复杂的属性，我们需要单独处理 对象：association 集合：collection
javaType=""指定属性的类型
集合中的泛型信息，我们使用ofType获取
-->
<collection property="students" ofType="Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<result property="tid" column="tid"/>
</collection>
</resultMap>
2、按照查询嵌套处理
<select id="getTeacher2" resultMap="TeacherStudent2">
select *from mybatis.teacher where id = #{tid}
</select>
<resultMap id="TeacherStudent2" type="Teacher">
<!--复杂的属性需要单独处理
对象：association
集合：collection
-->
<collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id"/>
</resultMap>
<select id="getStudentByTeacherId" resultType="Student">
select * from mybatis.student where tid =#{tid}
</select>
小结
1、关联-association【多对一】
2、集合-collection 【一对多】
3、javaType ofType
javaType用来指定实体类中属性的类型（例：private List<Student> students; 类型就是ArrayList)
ofType 用来指定映射到List或者集合中的pojo类型，泛型中的约束类型！（例private List<Student> students; 就是Student）
注意点：1、保证SQL 的可读性，尽量保证通俗易懂
2、注意一对多和多对一中，属性名和字段的问题
3、如果问题不好排查错误，可以使用日志，建议使用Log4j