Mybatis之多对一处理

多对一处理

使用场景: 老师有多名学生 ,对于学生而言 就是多对一

pojo.Student

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

mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
​
    <properties resource="db.properties">
        <property name="username" value="root"/>
        <property name="password" value="123456"/>
    </properties>
    
    <settings>
        <!--标准的日志工厂实现-->
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>
​
​
    <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>
        <environment id="test">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=false&amp;useUnicode=true&amp;characterEncoding=utf8&amp;zeroDateTimeBehavior=convertToNull&amp;serverTimezone=GMT%2B8"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
​
    <mappers>
        <mapper class="com.kuang.mapper.TeacherMapper"/>
        <mapper class="com.kuang.mapper.StudentMapper"/>
    </mappers>
</configuration>

 

1. 按照嵌套结果处理

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.kuang.mapper.StudentMapper">
​
<resultMap id="StudentMap" type="com.kuang.pojo.Student">
        <id 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="getStudent" resultMap="StudentMap">
        select * from student
    </select>
​
    <select id="getTeacher" resultType="teacher">
        select  * from teacher where id = #{tid}
    </select>

2. 联表查询处理

<resultMap id="StudentTeacher" type="com.kuang.pojo.Student">
        <id 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>
​
    <select id="getStudent2" resultMap="StudentTeacher">
        select s.id sid,s.name sname,t.id tid,t.name tname from student s join teacher t on s.tid = t.id
    </select>