Mybatis——多对一与一对多

• 一、多对一
• • • • 1.association标签
      • 2.分步查询
• 二、一对多
• • • • 1.collection标签
      • 2.分步查询

数据库
teacher表

student表

例：多个学生被一个老师教，查询学生信息同时查询出教师信息

• 实体类

teacher

@Data
public class Teacher {
    private String tid;
    private String name;
}

student

@Data
public class Student {
    private String sid;
    private String name;
    private Teacher teacher;
}

• mapper

public interface StudentMapper {
    //查询所有学生的信息
    public List findAllStudent();
}

• xml

    
        
        
      
        
            
            
        
        
    

    
        SELECT
            s.sid,
            s.NAME sname,
            s.tid,
            t.NAME tname
        FROM
            student s,
            teacher t
        WHERe
            s.tid = t.tid
    

property：需要处理的多对映射关系属性名
javaType：该属性的类型

先查询出该学生的全部信息（包括教师id）
再通过教师id查询到教师信息

    
        
        
        
    

    
        select * from student
    

    
        select * from teacher where tid = #{id}
    

例：一个老师教多个学生，查询该老师时显示所教所有学生信息

• 实体类
  teacher

@Data
public class Teacher {
    private String tid;
    private String name;
    private List student;
}

student

@Data
public class Student {
    private String sid;
    private String name;
    private String tid;
}

• mapper

public interface TeacherMapper {
    public Teacher findTeacherById(String id);
}

• xml

    
        SELECT
            s.sid,
            s.NAME sname,
            s.tid,
            t.NAME tname
        FROM
            student s,
            teacher t
        WHERe
            t.tid = #{id}
        AND
            s.tid = t.tid
    
    
    
        
        
        
            
            
        
    

 
 
 
ofType：表示该属性对应的集合存储类型(List studnet集合代表的存储类型）
 
 
2.分步查询 


    
        select * from teacher where tid = #{id}
    

    
        
        
        
            
            
            
        
    

    
        select * from student where tid = #{id}