一对多处理

1.导包

2.实体类

Student

package com.xy.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;

@Data
@AllArgsConstructor
@NoArgsConstructor
@ToString
public class Student {
    private int id;
    private String name;
    //学生需要关联一个老师
    private int tid;
}

Teacher

package com.xy.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;

import java.util.List;

@Data
@AllArgsConstructor
@NoArgsConstructor
@ToString
public class Teacher {
    private int id;
    private String name;
    //一个老师拥有多个学生
    private List<Student> students;
}

3.接口

StudentMapper.java

package com.xy.dao;

import com.xy.pojo.Student;

import java.util.List;

public interface StudentMapper {
}

TeacherMapper.java

package com.xy.dao;

import com.xy.pojo.Student;
import com.xy.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

import java.util.List;

public interface TeacherMapper {
    //查找教师信息以及其对应的学生的信息
    Teacher getTeacher(@Param("tid") int id);
}

4.建立Mapper接口

StudentMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.xy.dao.StudentMapper">
</mapper>

TeacherMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.xy.dao.TeacherMapper">
    <!--按结果嵌套-->
    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid,s.name sname, t.name tname, t.id tid
        from mybatis.student s , mybatis.teacher t
        where s.tid = t.id and t.id = #{tid}
    </select>
    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"></result>
        <!--javaType 指定属性的类型
            集合中的泛型信息，我们使用ofType获取
        -->
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>
    <!--子查询-->
    <select id = "getTeacher2" resultMap="TeacherStudent2">
        select * from mybatis.teacher where id = #{tid}
    </select>
    <!--选出来要查询的老师id，得到查询结果，这个是一个集合所以要写javaType-->
    <resultMap id="TeacherStudent2" type="Teacher">
        <collection property="students" javaType="ArrayList" ofType="Stundet" select="getStudentByTeacherID" column="id"/>
    </resultMap>
    <select id = "getStudentByTeacherID" resultType="Student">
        select * from mybatis.student where  tid = #{tid}
    </select>
</mapper>

5.在核心配置文件中绑定注册mapper

<mappers>
    <!--mapper不能用通配符 *-->
    <mapper resource="xiaoqi/dao/TeacherMapper.xml"/>
    <mapper resource="xiaoqi/dao/StudentMapper.xml"/>
</mappers>

6.测试

复杂查询小结

关联-association【多对一】

集合-collection【一对多】

javaType ofType

• javaType 用来指定属性实体类中的类型

• ofType 用来指定映射到List或者集合中的pojo类型，泛型中的约束类型

注意点：

• 保证sql的可读性，尽量保证通俗易懂
• 注意一对多和多对一中，属性和字段的问题
• 如果问题不好排查错误，可以使用日志，建议使用Log4J