多对一的处理

1、需求：查询所有的学生信息，以及对应的老师的信息

接口StudentMapper.java新增方法

/**
 * @Description TODO
 * @Author Administrator
 * @Date 2020/11/28 13:42
 */
public interface StudentMapper {

    /**
     * 查询所有的学生信息，以及对应的老师的信息
     * 按照查询嵌套
     * @return 学生信息
     */
    public List<Student> getStudent();

    /**
     * 查询学生所有信息
     * 按照结果嵌套
     * @return
     */
    public List<Student> getStudent2();

}

mapper文件StudentMapper.xml修改

(1)按照查询嵌套

<?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">

    <!--1、按照查询嵌套-->
    <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-->
        <!--对象使用association，集合使用collection-->
        <association property="teacher" column="tid" javaType="teacher" select="getTeacher"/>
    </resultMap>

    <select id="getTeacher" resultType="teacher">
        select * from teacher where id = #{id}
    </select>
</mapper>

(2)按照结果嵌套处理

<?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">
    
    <!--按照结果嵌套处理-->
    <select id="getStudent2" resultMap="StudentTeacher2">
        select s.id sid,s.name sname,t.id tid,t.name tname
        from student s, teacher t
        where t.id = s.tid
    </select>

    <resultMap id="StudentTeacher2" type="student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher">
            <result property="id" column="tid"/>
            <result property="name" column="tname"/>
        </association>
    </resultMap>

</mapper>

mybatis-config.xml注册mapper

<?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">
<!--核心配置文件-->
<configuration>
    <!--引入外部配置文件-->
    <properties resource="db.properties">
        <property name="username" value="root"/>
        <property name="password" value="13256"/>
    </properties>

    <!--设置-->
    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>

    <!--类型别名-->
    <typeAliases>
        <package name="com.kuang.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>

    <!--注册mapper-->
    <mappers>
        <!--teacher对应的mapper-->
        <mapper resource="com/kuang/dao/TeacherMapper.xml"/>
        <!--student对应的mapper-->
        <mapper class="com.kuang.dao.StudentMapper"/>
    </mappers>

</configuration>

测试

@Test
public void test02(){

    SqlSession sqlSession = MybatisUtils.getSqlSession();
    StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
    List<Student> students = studentMapper.getStudent();
    students.forEach(System.out::println);

    sqlSession.close();

}

结果

Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.
Class not found: org.jboss.vfs.VFS
JBoss 6 VFS API is not available in this environment.
Class not found: org.jboss.vfs.VirtualFile
VFS implementation org.apache.ibatis.io.JBoss6VFS is not valid in this environment.
Using VFS adapter org.apache.ibatis.io.DefaultVFS
Find JAR URL: file:/E:/code/Java/mybatis-study/mybatis-05/target/classes/com/kuang/pojo
Not a JAR: file:/E:/code/Java/mybatis-study/mybatis-05/target/classes/com/kuang/pojo
Reader entry: Student.class
Reader entry: Teacher.class
Listing file:/E:/code/Java/mybatis-study/mybatis-05/target/classes/com/kuang/pojo
Find JAR URL: file:/E:/code/Java/mybatis-study/mybatis-05/target/classes/com/kuang/pojo/Student.class
Not a JAR: file:/E:/code/Java/mybatis-study/mybatis-05/target/classes/com/kuang/pojo/Student.class
Reader entry: ����   4 ^
Find JAR URL: file:/E:/code/Java/mybatis-study/mybatis-05/target/classes/com/kuang/pojo/Teacher.class
Not a JAR: file:/E:/code/Java/mybatis-study/mybatis-05/target/classes/com/kuang/pojo/Teacher.class
Reader entry: ����   4 L
Checking to see if class com.kuang.pojo.Student matches criteria [is assignable to Object]
Checking to see if class com.kuang.pojo.Teacher matches criteria [is assignable to Object]
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
Opening JDBC Connection
Created connection 525968792.
==>  Preparing: select * from student 
==> Parameters: 
<==    Columns: id, name, tid
<==        Row: 1, 小明, 1
====>  Preparing: select * from teacher where id = ? 
====> Parameters: 1(Integer)
<====    Columns: id, name
<====        Row: 1, 秦老师
<====      Total: 1
<==        Row: 2, 小红, 1
<==        Row: 3, 小张, 1
<==        Row: 4, 小李, 1
<==        Row: 5, 小王, 1
<==      Total: 5
Student(id=1, name=小明, teacher=Teacher(id=1, name=秦老师))
Student(id=2, name=小红, teacher=Teacher(id=1, name=秦老师))
Student(id=3, name=小张, teacher=Teacher(id=1, name=秦老师))
Student(id=4, name=小李, teacher=Teacher(id=1, name=秦老师))
Student(id=5, name=小王, teacher=Teacher(id=1, name=秦老师))
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@1f59a598]
Returned connection 525968792 to pool.

Process finished with exit code 0