多对一处理

复杂查询环境搭建

• 多（学生）对一（老师）

CREATE TABLE `teacher` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;

INSERT INTO teacher(`id`, `name`) VALUES (1, ‘秦老师‘); 

CREATE TABLE `student` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  `tid` INT(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fktid` (`tid`),
  CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;
INSERT INTO `student` (`id`, `name`, `tid`) VALUES (1, ‘小明‘, 1); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES (2, ‘小红‘, 1); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES (3, ‘小张‘, 1); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES (4, ‘小李‘, 1); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES (5, ‘小王‘, 1);

• 建立Student，Teacher实体类，其属性与数据库对应。StudentMapper.java，StudentMapper.xml等，并在mybatis-config中导入

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

按照查询嵌套处理StudentMapper.xml

<?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="com.dao.StudentMapper">
    <resultMap id="Student_Teacher" type="com.pojo.Student">
        <association property="teacher" column="tid" javaType="com.pojo.Teacher" select="getTeacher"/>
    </resultMap>

    <select id="getStudent" resultMap="Student_Teacher">
        select *
        from student
    </select>

    <select id="getTeacher" resultType="com.pojo.Teacher">
        select * from teacher where id = #{id}
    </select>
</mapper>

按照结果嵌套处理

<?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="com.dao.StudentMapper">

    <resultMap id="Student_Teacher"  type="com.pojo.Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="com.pojo.Teacher">
            <result property="name" column="tname"/>
            <result property="id" column="tid"/>
        </association>
    </resultMap>
    <select id="getStudent" resultMap="Student_Teacher">
        select s.id sid, s.name sname, t.name tname,t.id tid
        from student s,
             teacher t
        where s.tid = t.id;
    </select>

</mapper>

一对多处理

基本类

public class Teacher {
    private int id;
    private String name;
    private List<Student> students;
}
public class Student {
    private int id;
    private String name;
    private int tid;
}
public interface TeacherMapper {
    Teacher getTeacher(@Param("tid") int id);
}

按照结果嵌套处理TeacherMapper.xml

<?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="com.dao.TeacherMapper">
    <select id="getTeacher" resultMap="Teacher_Student">
        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 = #{tid}
    </select>

    <resultMap id="Teacher_Student" type="com.pojo.Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <collection property="students" ofType="com.pojo.Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>
</mapper>

按照查询嵌套处理

<?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="com.dao.TeacherMapper">

    <resultMap id="Teacher_Student" type="com.pojo.Teacher">
        <collection property="students" javaType="ArrayList" ofType="com.pojo.Student" select="getStudentByTeacherId" column="id"/>
    </resultMap>

    <select id="getTeacher" resultMap="Teacher_Student">
        select *
        from teacher
        where id = #{tid}
    </select>

    <select id="getStudentByTeacherId" resultType="com.pojo.Student">
        select *
        from student
        where tid = #{tid}
    </select>
</mapper>

小结

关联-association 【多对一】

集合-collection 【一对多】

javaType & ofType

• javaType用来指定实体类中属性的类型
• 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

面试高频

• Mysql引擎
• innoDB底层原理
• 索引
• 索引优化

复杂查询，一对多，多对一，面试高频