20-多对一的处理

复杂查询环境搭建

1.导入lombok

2.新建实体类 Teacher Student (package com.tian.dao;)

@Data
public class Teacher {
    private int id;
    private String name;
}



@Data
public class Student {
    private int id;
    private String name;

    //学生需要关联一个老师
    private Teacher teacher;
}

3.建立Mapper接口

public interface TeacherMapper {
    @Select("select  * from mybatis.teacher where id=#{tid}")
    Teacher getTeacher(@Param("tid") int id);
}




public interface StudentMapper {
}

4.建立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.tian.dao.TeacherMapper">

<!--<select id="getTeacher" parameterType="int" resultType="Teacher">-->
<!--    select  * from mybatis.teacher where id=#{tid};-->
<!--</select>-->
</mapper>
    
    
    
    
    <?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.tian.dao.StudentMapper">


</mapper>

5.在核心配置文件中注册我们的Mapper接口或者文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<!--核心配置环境-->
<configuration>
<!--  引入外部配置文件  优先使用  -->
    <properties resource="db.properties" />

<settings>
    <setting name="logImpl" value="STDOUT_LOGGING"/>
</settings>

<!--可以给实体类取别名-->
    <typeAliases>
        <package name="com.tian.pojo"/>
    </typeAliases>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>

<mappers>
    <mapper class="com.tian.dao.TeacherMapper"/>
    <mapper class="com.tian.dao.StudentMapper"/>
</mappers>
</configuration>

6.测试查询是否能够成功

 @Test
    public void getUserByLimit(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher(1);
        System.out.println(teacher);
        sqlSession.close();
    }

多对一的处理

按照查询嵌套处理

<?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.tian.dao.StudentMapper">

<!--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 s;
</select>
    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!-- 复杂的属性，我们需要单独处理 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>
</mapper>

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>