MyBatis一对多的左连接查询、分步查询以及插入和删除操作

例如有两张表，分别是客户表和订单表，一个客户有多个订单，一个订单属于一个客户。

两个实体类Customer   Order 如下：

package com.itlike.domain;

import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

import java.util.ArrayList;
import java.util.List;

@Setter@Getter@ToString
public class Customer {
    private Integer cust_id;
    private String cust_name;
    private String cust_profession;
    private String cust_phone;
    private String email;
    private List orders = new ArrayList<>();
}

package com.itlike.domain;

import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

@Setter@Getter@ToString
public class Order {
    private Integer order_id;
    private String order_name;
    private String order_num;
    private Customer customer;
}

 数据库中的表如下：

1.一对多的左连接查询

在CustomerMapper接口中定义方法

//查询所有的客户
    public List getAllCustomers();

在CustomerMapper.xml中注入SQL

collection中的  javatype 表示  orders是一个list集合    oftype表示list集合中装的都是Order类型

        SELECT * from `customer` as c LEFT JOIN `order` as o ON c.cust_id=o.cust_id;
    

    
        
        
        
        
        

      
          
          
          
      

    

2.一对多的分步查询

CustmoerMapper.xml中的SQL注入

    
        select * from `customer`;
    

    
        
        
        
        
        

        

        

    

OrderMapper.xml中的SQL注入

  
        select * from `order` where cust_id=#{id};
    

3.一对多的插入操作

在CustomerMapper接口中定义方法

//添加客户
    public void insertCustomer(Customer customer);

在CustomerMapper.xml中注入插入客户信息的SQL    先插入客户信息

 
    
        insert into `customer`(cust_name,cust_profession,cust_phone,email)
         values (#{cust_name},#{cust_profession},#{cust_phone},#{email})
    

在OrderMapper.xml中注入插入订单信息的SQL    再插入订单信息

  
        insert into `order` (order_name,order_num,cust_id )
        values (#{order_name},#{order_num},#{customer.cust_id})
    

最后还要在order表中写入外键   即对应的客户编号   更新客户和订单的关系

在OrderMapper.xml中注入插入更新关系的SQL

 
        update `order` set cust_id = ${custId} where order_id = ${orderId}
    

测试类代码：

 public void test6(){
        SqlSession sqlSession = MyBatisUtils.openSession();
        CustomerMapper customerMapper = sqlSession.getMapper(CustomerMapper.class);
        OrderMapper orderMapper = sqlSession.getMapper(OrderMapper.class);
        Customer customer = new Customer();
        customer.setCust_name("新客户");

        Order order1 = new Order();
        order1.setOrder_name("订单1");

        Order order2 = new Order();
        order2.setOrder_name("订单2");

        customer.getOrders().add(order1);
        customer.getOrders().add(order2);

        //保存数据
        customerMapper.insertCustomer(customer);
        orderMapper.insertOrder(order1);
        orderMapper.insertOrder(order2);


        //更新关系
        for (Order order : customer.getOrders()) {
            orderMapper.updateCustId(order.getOrder_id(),customer.getCust_id());
        }

        sqlSession.commit();
        sqlSession.close();
    }

4.一对多的删除操作

必须先得打破外键的关系     然后再分别删除客户表和订单表中的信息

（1）在订单表Order中打破外键

OrderMapper接口中定义方法

//打破外键关系
    public void updateRelationCustomer(Integer custId);

OrderMapper.xml中的SQL

        update `order` set cust_id =null where cust_id=#{custId}

测试类代码：

public void test7(){
        SqlSession sqlSession = MyBatisUtils.openSession();
        CustomerMapper customerMapper = sqlSession.getMapper(CustomerMapper.class);
        OrderMapper orderMapper = sqlSession.getMapper(OrderMapper.class);
        //一对多删除之前   要先打破外键关系
        orderMapper.updateRelationCustomer(18);
        customerMapper.deleteCustomer(18);
        sqlSession.commit();
        sqlSession.close();
    }

在多对多的关系中，处理方法和一对多相似

查询时可以左连接查询    也可以分步查询          

 （重点是查询时SQL的写法               左连接查询时                表一左连接中间表左连接表二 ）

      SELECT * from `teacher` as t LEFT JOIN `stu_teacher_rel` as str ON t.teacher_id= str.teacher_id
      LEFT JOIN student as s ON str.stu_id = s.stu_id;

插入时也分别插入两个表的信息    然后建立中间表的对应关系  

删除时也应该先打破中间表的关系   然后再分别删除