select t.t_id,t.t_name,s.s_id,s_name
from t_teacher t,t_inner i,t_student s
where t.t_id=i.t_id and i.s_id=s.s_id and t.t_id
=1

select s.s_id,s_name,t.t_id,t.t_name
from t_student s,t_inner i,t_teacher t
where s.s_id=i.s_id and i.t_id=t.t_id and s.s_id
=1

6.4.2 创建Teacher和Student实体

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Teacher {
private Integer tId;
private String tName;
//表示一个老师教过多个学生
private List<Student> studentList;
}


@Data
@NoArgsConstructor
@AllArgsConstructor
public class Student {
private Integer sId;
private String sName;
//表示一个学生被多个老师教过
private List<Teacher> teacherList;
}

6.4.3 创建TeacherMapper和 StudentMapper接口

public interface TeacherMapper {
//根据ID查询老师信息
public Teacher findTeacherById(Integer tId);
}
public interface StudentMapper {
//根据ID查询学生信息
public Student findStudentById(Integer sId);
}

6.4.4 配置TeacherMapper.xml和 StudentMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper
3.0//EN"
"https://mybatis.org/dtd/mybatis-3-
mapper.dtd">
<mapper
namespace="com.lzw.mapper.TeacherMapper">
<resultMap id="teacherResultMap"
type="Teacher">
<id property="tId" column="t_id"></id>
<result property="tName" column="t_name">
</result>
<collection property="studentList"
ofType="Student">
<id property="sId" column="s_id"></id>
<result property="sName"
column="s_name"></result>
</collection>
</resultMap>
<select id="findTeacherById"
parameterType="int"
resultMap="teacherResultMap">
select t.t_id,t.t_name,s.s_id,s_name
from t_teacher t,t_inner i,t_student s
where t.t_id=i.t_id and i.s_id=s.s_id and
t.t_id =#{tId}
</select>
</mapper>
------------------------------------------------
------------------------------------
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper
3.0//EN"
"https://mybatis.org/dtd/mybatis-3-
mapper.dtd">
<mapper
namespace="com.lzw.mapper.StudentMapper">
<resultMap id="studentResultMap"
type="Student">
<id property="sId" column="s_id"></id>
<result property="sName" column="s_name">
</result>
<collection property="teacherList"
ofType="Teacher">
<id property="tId" column="t_id"></id>
<result property="tName"
column="t_name"></result>
</collection>
</resultMap>
<select id="findStudentById"
parameterType="int"
resultMap="studentResultMap">
select s.s_id,s_name,t.t_id,t.t_name
from t_student s,t_inner i,t_teacher t
where s.s_id=i.s_id and i.t_id=t.t_id and
s.s_id =#{sId}

</select>
</mapper>

6.4.5 测试结果

@Test
public void testDemo(){
//使用工具类获取SqlSession对象
SqlSession sqlSession =
SqlSessionUtil.openSession();
//基于接口获取实现类对象(代理对象)
TeacherMapper teacherMapper =
sqlSession.getMapper(TeacherMapper.class);
StudentMapper studentMapper =
sqlSession.getMapper(StudentMapper.class);
//通过多态调用方法
Teacher teacher =
teacherMapper.findTeacherById(1);
Student student =
studentMapper.findStudentById(1);
//输出结果
System.out.println(teacher);
System.out.println(student);
//释放资源
sqlSession.close();
}

七.MyBatis动态SQL

7.1 动态SQL需求和简介

public interface EmployeeMapper {
//根据条件查询用户信息
public List<Employee>
findByCondition(Map<String,Object> map);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper
3.0//EN"
"https://mybatis.org/dtd/mybatis-3-
mapper.dtd">
<mapper
namespace="com.lzw.mapper.EmployeeMapper">
<!--
if标签:判断这个SQL语句片段是否要加入整个SQL语
句
true加入整个SQL语句中
false不加入整个SQL语句中
where标签:判断是否追加where关键字,会自动去除
多余的 and or关键字
-->
<select id="findByCondition"
resultType="Employee">
select * from t_emp
<where>
<if test="empName !=null">
emp_name = #{empName}
</if>
<if test="empSalary !=null">
and emp_salary = #{empSalary}
</if>
</where>
</select>

</mapper>

@Test
public void testDemo(){
//使用工具类获取SqlSession对象
SqlSession sqlSession =
SqlSessionUtil.openSession();
//基于接口获取实现类对象(代理对象)
EmployeeMapper employeeMapper =
sqlSession.getMapper(EmployeeMapper.class);
//通过多态调用方法
Map<String,Object> map=new HashMap<>();
//map.put("empName","张三");
//map.put("empSalary",200.33);
List<Employee> employeeList =
employeeMapper.findByCondition(map);
//输出结果
System.out.println(employeeList);
//释放资源
sqlSession.close();
}