Mybatis一对多与多对一查询处理详解
作者：KittyGuy
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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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