SSM框架（六）Mybatis实现一对多和多对一查询

<!-- https://mvnrepository.com/artifact/org.projectlombok/lombok -->
<dependency>
 <groupId>org.projectlombok</groupId>
 <artifactId>lombok</artifactId>
 <version>1.16.10</version>
</dependency>

@Data
public class Student {
   private int id;
   private String name;
   //多个学生可以是同一个老师，即多对一
   private Teacher teacher;
}

//获取所有学生及对应老师的信息
public List<Student> getStudents();

<?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="top.hcode.mapper.StudentMapper">

   <!--
   需求：获取所有学生及对应老师的信息 -->

   <select id="getStudents" resultMap="StudentTeacher">
    select * from student
   </select>
   <resultMap id="StudentTeacher" type="Student">
       <!--association关联属性 property属性名 javaType属性类型 column在多的一方的表中的列名-->
       <association property="teacher"  column="tid" javaType="Teacher" select="getTeacher"/>
   </resultMap>
   <!--
   这里传递过来的id，只有一个属性的时候，下面可以写任何值
   association中column多参数配置：
       column="{key=value,key=value}"
       其实就是键值对的形式，key是传给下个sql的取值名称，value是片段一中sql查询的字段名。
   -->
   <select id="getTeacher" resultType="teacher">
      select * from teacher where id = #{id}
   </select>

</mapper>

<!--
按查询结果嵌套处理
思路：
   1. 直接查询出结果，进行结果集的映射
-->
<select id="getStudents" resultMap="StudentTeacher" >
  select s.id sid, s.name sname , t.name tname
  from student s,teacher t
  where s.tid = t.id
</select>

<resultMap id="StudentTeacher" type="Student">
   <id property="id" column="sid"/>
   <result property="name" column="sname"/>
   <!--关联对象property 关联对象在Student实体类中的属性-->
   <association property="teacher" javaType="Teacher">
       <result property="name" column="tname"/>
   </association>
</resultMap>

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

@Data 
public class Teacher {
   private int id;
   private String name;
   //一个老师多个学生
   private List<Student> students;
}

//获取指定老师，及老师下的所有学生
public Teacher getTeacher(int id);

<mapper namespace="top.hcode.mapper.TeacherMapper">

   <!--
   思路:
       1. 从学生表和老师表中查出学生id，学生姓名，老师姓名
       2. 对查询出来的操作做结果集映射
           1. 集合的话，使用collection！
               JavaType和ofType都是用来指定对象类型的
               JavaType是用来指定pojo中属性的类型
               ofType指定的是映射到list集合属性中pojo的类型。
   -->
   <select id="getTeacher" resultMap="TeacherStudent">
      select s.id sid, s.name sname , t.name tname, t.id tid
      from student s,teacher t
      where s.tid = t.id and t.id=#{id}
   </select>
    //查询出来的结果映射为对应的实体类集合
   <resultMap id="TeacherStudent" type="Teacher">
       <result  property="name" column="tname"/>
       <collection property="students" ofType="Student">
           <result property="id" column="sid" />
           <result property="name" column="sname" />
           <result property="tid" column="tid" />
       </collection>
   </resultMap>
</mapper>

<select id="getTeacher" resultMap="TeacherStudent">
select * from teacher where id = #{id}
</select>
<select id="getStudentByTeacherId" resultType="Student">
  select * from student where tid = #{id}
</select>
//根据查出来的指定的老师id去查所有学生中对应的老师id相同的学生。
<resultMap id="TeacherStudent" type="Teacher">
   <!--column是一对多的外键 , 写的是一的主键的列名-->
   <collection property="students" javaType="ArrayList" ofType="Student" column="id" select="getStudentByTeacherId"/>
</resultMap>