第十三章MyBatis高级映射

多对一映射

创建数据表

• student是主表
• class_id关联class表的id

class表

student表

创建pojo

Class类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Class {

  private Long id;
  private String name;
  private List students;
}

Student类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {

  private Long id;
  private String name;
  private Long classId;
  private Class clazz;
}

映射方式

1. 级联映射

StudentMapper写入如下代码

Student selectByIdResult(Long id);

StudentMapper.xml写入如下代码

    s.id,
    s.name,
    s.class_id,
    c.id as classId,
    c.name as className



    
    
    
    
    



    select
    
    from student s left join class c on s.class_id = c.id
    where s.id=#{id}

2. association映射

StudentMapper写入如下代码

Student selectByIdAssociation(Long id);

StudentMapper.xml写入如下代码

    s.id,
    s.name,
    s.class_id,
    c.id as classId,
    c.name as className


    
    
    
    
        
        
    


    select 
    
    from student s left join class c on s.class_id=c.id
    where s.id = #{id}

3. 分步查询

• 上述两种方法都是一条sql完成
• 分步查询需要俩条sql，可复用支持延迟加载

StudentMapper.xml写入如下代码

Student selectByIdStep(Long id);

ClassMapper写入如下代码

Class selectById(Long id);

StudentMapper.xml写入如下代码

    id,
    name,
    class_id


    
    
    
    


    select 
    
    from student where id = #{id}

ClassMapper.xml写入如下代码

    id,
    name


    select
    
    from class
    where id=#{id}

上述俩种查询每次都要查询2条sql，有时我们不需要查询2条。并且联表查询会出现笛卡尔现象，性能很差

这可以是用延迟加载来提高性能，用到属性的时候查询不用就不会查询

局部设置 fetchType=“lazy” 。只会作用域这条sql

全局开启延迟加载

局部关闭使用 fetchType=“eager”

一对多映射

创建pojo

Class类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Class {

  private Long id;
  private String name;
  private List students;
}

Student类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {

  private Long id;
  private String name;
  private Long classId;
  private Class clazz;
}

映射方式

1. collection映射

ClassMapper写入如下代码

Class selectCollection(Long id);

ClassMapper.xml写入如下代码

    c.id,
    c.name,
    s.id as studentId,
    s.name as studentName,
    s.class_id as classId


    
    
    
        
        
        
    


    select
    
    from class c left join student s on c.id=s.class_id
    where c.id=#{id}

2. 分步查询

ClassMapper写入如下代码

Class selectByIdStep(Long id);

StudentMapper写入如下代码

Student selectById(Long classId);

ClassMapper.xml写入如下代码

    id,
    name


    
    
    


    select
    
    from class where id=#{id}

StudentMapper.xml写入如下代码

    id,
    name,
    class_id


    select
    
    from student where class_id = #{classId}

同理可以设置延迟加载