一对一关联

<?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">
<!-- 为这个mapper指定一个唯一的namespace，namespace的值习惯上设置成包名+sql映射文件名，这样就能够保证namespace的值是唯一的
例如namespace="me.gacl.mapping.classMapper"就是me.gacl.mapping(包名)+classMapper(classMapper.xml文件去除后缀)
 -->
<mapper namespace="me.gacl.mapping.classMapper">

    <!-- 
        根据班级id查询班级信息(带老师的信息)
        ##1. 联表查询
        SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1;
        
        ##2. 执行两次查询
        SELECT * FROM class WHERE c_id=1;  //teacher_id=1
        SELECT * FROM teacher WHERE t_id=1;//使用上面得到的teacher_id
     -->

    <!-- 
    方式一：嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集
             封装联表查询的数据(去除重复的数据)
        select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=1
    -->
    <select id="getClass" parameterType="int" resultMap="ClassResultMap">
        select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
    </select>
    <!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
    <resultMap type="me.gacl.domain.Classes" id="ClassResultMap">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" javaType="me.gacl.domain.Teacher">
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
    </resultMap>
    
    <!-- 
    方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型
        SELECT * FROM class WHERE c_id=1;
        SELECT * FROM teacher WHERE t_id=1   //1 是上一个查询得到的teacher_id的值
    -->
     <select id="getClass2" parameterType="int" resultMap="ClassResultMap2">
        select * from class where c_id=#{id}
     </select>
     <!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
     <resultMap type="me.gacl.domain.Classes" id="ClassResultMap2">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" select="getTeacher"/>
     </resultMap>
     
     <select id="getTeacher" parameterType="int" resultType="me.gacl.domain.Teacher">
        SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
     </select>

</mapper>

一对多关联

<!-- 
        根据classId查询对应的班级信息,包括学生,老师
     -->
    <!-- 
    方式一: 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集
    SELECT * FROM class c, teacher t,student s WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND  c.c_id=1
     -->
    <select id="getClass3" parameterType="int" resultMap="ClassResultMap3">
        select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and  c.c_id=#{id}
    </select>
    <resultMap type="me.gacl.domain.Classes" id="ClassResultMap3">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher">
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
        <!-- ofType指定students集合中的对象类型 -->
        <collection property="students" ofType="me.gacl.domain.Student">
            <id property="id" column="s_id"/>
            <result property="name" column="s_name"/>
        </collection>
    </resultMap>
    
    <!-- 
        方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型
            SELECT * FROM class WHERE c_id=1;
            SELECT * FROM teacher WHERE t_id=1   //1 是上一个查询得到的teacher_id的值
            SELECT * FROM student WHERE class_id=1  //1是第一个查询得到的c_id字段的值
     -->
     <select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
        select * from class where c_id=#{id}
     </select>
     <resultMap type="me.gacl.domain.Classes" id="ClassResultMap4">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher" select="getTeacher2"></association>
        <collection property="students" ofType="me.gacl.domain.Student" column="c_id" select="getStudent"></collection>
     </resultMap>
     
     <select id="getTeacher2" parameterType="int" resultType="me.gacl.domain.Teacher">
        SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
     </select>
     
     <select id="getStudent" parameterType="int" resultType="me.gacl.domain.Student">
        SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
     </select>