mybatis实现多表一对一，一对多，多对多关联查询

1、一对一
关键字：association
作用：针对pojo对象属性的映射
      property：pojo的属性名
      javaType：pojo类名
(1) 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集

  	
  	
  	
  	
	
  	
  		
		
  		
  		
  	
 
  
  	select p.id as personId,p.name,p.sex,p.age,c.*
  	from tbl_person p,tbl_card c where p.card_id=c.id;
   

(2) 嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型
    column="引入执行另外定制sql方法的参数值（外键）"
    select="执行定制sql方法名"
PersonMapper2.xml

  	
  	
  	
  	
  	
  	


	select * from tbl_person;

CardMapper.xml

  	
  	


  	select c.id as card_id,c.code from tbl_card c 
	where c.id=#{id};


	
  		
  		
  		
  		
  	


	SELECT c.id AS card_id, c.code, p.* 
  	FROM tbl_card c,tbl_person p WHERE c.id=p.card_id;

2 、一对多
mybatis如何实现一对多的实现？(学生与班级)
(1) 嵌套结果:
ClazzMapper.xml

	
	
	


	
		
		
		
		
	


	SELECT c.*, s.id AS stu_id,s.name,s.sex,s.age
	FROM tbl_clazz c LEFT JOIN tbl_student s
	ON c.id=s.clazz_id;

StudentMapper.xml

	
	
	
	
 

	
		
		
		
	
 

	SELECT s.*,c.id AS cls_id,c.clazz_name,c.code
	FROM tbl_student s,tbl_clazz c WHERE s.clazz_id=c.id;

(2) 嵌套查询：
ClazzMapper.xml

	
	
	


	
	


	select * from tbl_clazz;

StudentMapper.xml

	
	
	
	
 

	
		
		
		
	
 

	SELECT s.*,c.id AS cls_id,c.clazz_name,c.code
	FROM tbl_student s,tbl_clazz c WHERE s.clazz_id=c.id;



  	select * from tbl_student where clazz_id=#{id};		

3、多对多
商品表、订单表之间就是以多对多关联
商品与订单的关系表
描述多对多的数据表实现
(1)商品pojo:
Article.java

public class Article implements Serializable {
	private Integer articleId;
	private String name;
	private Double price;
	private String remark;
	private List orders;
	省略setter/gettera方法
}

(2)商品表映射：
ArticleMapper.xml

	
		
		
		
		
	
	
		
		
	
	
	
		select * from tb_article  where id 
		in (select article_id from tb_item where order_id=#{id}) 
	
	
		select * from tb_article  where id=#{id}
	

(3)订单pojo：
Order.java

public class Order {
	private Integer orderid;
	private String code;
	private Double total;
	private List
 articles;
	省略setter/getter方法
}

(4)订单表映射：
OrderMapper.xml

	
		
		
		
	
	
		
		
	
	
	
		select * from tb_order  where id 
		in (select order_id from tb_item where article_id=#{id})