要点

• 主要还是结果集映射（resultMap）
• association标签： 一个复杂类型的关联；许多结果将包装成这种类型（JavaBean）嵌套结果映射，关联可以是 resultMap 元素，或是对其它结果映射的引用
• collection标签： 一个复杂类型的集合（List）嵌套结果映射，集合可以是resultMap元素，或是对其它结果映射的引用

一对多（association）

数据库结构

tid是student的外键，是teacher表的id

JavaBean

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

mapper.java

public interface StudentMapper { List<Student> getStudent(); List<Student> getStudent2();}

Student类里面有teacher，要想查出Student中的Teacher就需要映射。

方法有二

(1)

<select id="getStudent" resultMap="StudentTeacher"> select * from mybatis.student </select> <resultMap id="StudentTeacher" type="Student"> <result property="id" column="id"/> <result property="name" column="name"/> <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/> </resultMap> <!--子查询--> <select id="getTeacher" resultType="Teacher"> select * from mybatis.teacher where id=#{Anything} </select>

(2)

<select id="getStudent2" resultMap="StudentTeacher2"> select * from mybatis.student as s ,mybatis.teacher as t where s.tid=t.id </select> <resultMap id="StudentTeacher2" type="Student"> <result property="id" column="id"/> <result property="name" column="name"/> <association property="teacher" javaType="Teacher"> <result property="id" column="tid"/> <result property="name" column="name"/> </association> </resultMap>

多对一

JavaBean

public class Teacher2 { private int id; private String name; //一个老师对应多个学生 private List<Student2> students;}public class Student2 { private int id; private String name; private int tid;}

mapper.java

public interface TeacherMapper2 { List<Teacher2> getTeacher(@Param("id") int id);}

mapper.xml

<?xml version="1.0" encoding="GBK" ?><!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.mybatis.DAO.TeacherMapper2"> <select id="getTeacher" parameterType="int" resultMap="teacherS"> select s.id, s.name, s.tid,t.id as tid, t.name as tname from mybatis.student as s ,mybatis.teacher as t where s.tid=t.id and t.id=#{id} </select> <resultMap id="teacherS" type="teacher2"> <result property="id" column="tid"/> <result property="name" column="tname"/> <collection property="students" ofType="student2"> <result property="id" column="id"/> <result property="name" column="name"/> <result property="tid" column="tid"/> </collection> </resultMap></mapper>

小结

• 一对多和多对一区别不大
• 其实就是association（类）和collection（集合）的区别
• 还有ofType和javaType的区别
• 如果查询结果不符合预期，请设置别名试一试

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