Mybatis中表之间的关联关系

1.关联关系是有方向的.

1)一对多关联:一个老师可以教多个学生,多个学生只有一个老师来教,站在老师方,就是一对多关联.
2)多对一关联:一个老师可以教多个学生,多个学生只有一个老师来教,站在学生方,就是多对一关联.
3)一对一关联:一个老师辅导一个学生,一个学生只请教一个老师.学生和老师是一对一.
4)多对多关联:园区划线的车位和园区的每一辆车.任意一个车位可以停任意一辆车.任意一车辆车可以停在任意一个车位上.

2.一对多关联关系

客户和订单就是典型的一对多关联关系.
一个客户名下可以有多个订单.
客户表是一方,订单表是多方.客户一中持有订单的集合.
使用一对多的关联关系,可以满足查询客户的同时查询该客户名下的所有订单.

客户表：

订单表：

Customer实体类：

package com.xin.entity;

import java.util.List;

public class Customer {
    //customer表中的三个列
    private Integer id;
    private  String name;
    private Integer age;
    //该客户名下的所有订单集合
    private List<Orders> ordersList;

    public Customer() {
    }

    public Customer(Integer id, String name, Integer age, List<Orders> ordersList) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.ordersList = ordersList;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public List<Orders> getOrdersList() {
        return ordersList;
    }

    public void setOrdersList(List<Orders> ordersList) {
        this.ordersList = ordersList;
    }

    @Override
    public String toString() {
        return "Customer{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", age=" + age +
                ", ordersList=" + ordersList +
                '}';
    }
}

CustomerMapper接口：

import com.xin.entity.Customer;

public interface CustomerMapper {
    //根据客户的id查询所有用户信息并同时查询该客户下的所有订单
    Customer getById(Integer id);
}

CustomerMapper.xml文件：

<?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.xin.mapper.CustomerMapper">
    <!--
         //根据客户的id查询所有用户信息并同时查询该客户下的所有订单
         Customer getById(Integer id);
         customer表： //customer表中的三个列
                        private Integer id;
                        private  String name;
                        private Integer age;
                        //该客户名下的所有订单集合
                        private List<Orders> ordersList;
         orders表：public Integer id;
                  public String orderNumber;
                  public String orderPrice;
    -->
    <resultMap id="customermap" type="customer">
        <!--主键绑定-->
        <id property="id" column="cid"></id>
        <!--非主键绑定-->
        <result property="name" column="name"></result>
        <result property="age" column="age"></result>
        <!--             //该客户名下的所有订单集合
                         private List<Orders> ordersList;
                         orders表：public Integer id;
                                  public String orderNumber;
                                  public String orderPrice;
         -->
        <collection property="ordersList" ofType="orders">
            <!--主键绑定-->
            <id property="id" column="oid"></id>
            <!--非主键绑定-->
            <result property="orderNumber" column="orderNumber"></result>
            <result property="orderPrice" column="orderPrice"></result>
        </collection>
    </resultMap>
    <select  id="getById" parameterType="int" resultMap="customermap">
        select c.id cid,name,age,o.id oid,orderNumber,orderPrice,customer_id
        from customer c left join orders o on c.id=o.customer_id
        where c.id=#{id}
    </select>
</mapper>

测试：

@Test
    public void testGetById() {
        Customer customer=customerMapper.getById(1);
        System.out.println(customer);
    }

3.多对一关联关系.

订单和客户就是多对一关联.
站在订单的方向查询订单的同时将客户信息查出.
订单是多方,会持有一方的对象.客户是一方.

Orders1实体类：

package com.xin.entity;

public class Orders1 {
    public Integer id;
    public String orderNumber;
    public String orderPrice;
    public Customer customer;

    public Orders1() {
    }

    public Orders1(Integer id, String orderNumber, String orderPrice, Customer customer) {
        this.id = id;
        this.orderNumber = orderNumber;
        this.orderPrice = orderPrice;
        this.customer = customer;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getOrderNumber() {
        return orderNumber;
    }

    public void setOrderNumber(String orderNumber) {
        this.orderNumber = orderNumber;
    }

    public String getOrderPrice() {
        return orderPrice;
    }

    public void setOrderPrice(String orderPrice) {
        this.orderPrice = orderPrice;
    }

    public Customer getCustomer() {
        return customer;
    }

    public void setCustomer(Customer customer) {
        this.customer = customer;
    }

    @Override
    public String toString() {
        return "Orders1{" +
                "id=" + id +
                ", orderNumber='" + orderNumber + '\'' +
                ", orderPrice='" + orderPrice + '\'' +
                ", customer=" + customer +
                '}';
    }
}

Orders1Mapper接口:

package com.xin.mapper;

import com.xin.entity.Orders1;

public interface Orders1Mapper {
    Orders1 getById1(Integer id);
}

Orders1Mapper.xml文件

<?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.xin.mapper.Orders1Mapper">
    <!--
          Orders1Mapper getById(Integer id);
          orders1表：public Integer id;
                    public String orderNumber;
                    public String orderPrice;
                    public Customer customer;
          customer表：/customer表中的三个列
                    private Integer id;
                    private  String name;
                    private Integer age;
                    //该客户名下的所有订单集合
                    private List<Orders> ordersList;//这个不用写
    -->
    <resultMap id="orders1map" type="orders1">
        <id property="id" column="oid"></id>
        <result property="orderNumber" column="orderNumber"></result>
        <result property="orderPrice" column="orderPrice"></result>
        <association property="customer" javaType="customer">
            <id property="id" column="cid"></id>
            <result property="name" column="name"></result>
            <result property="age" column="age"></result>
        </association>
    </resultMap>
    <select id="getById1" parameterType="int" resultMap="orders1map">
        select o.id oid,orderNumber,orderPrice,customer_id,c.id cid,name,age
        from orders o inner join customer c on o.customer_id=c.id
        where o.id=#{id}
    </select>
</mapper>

测试：

@Test
    public void testGetById1() {
        Orders1 orders1=orders1Mapper.getById1(11);
        System.out.println(orders1);
    }

总结：无论是什么关联关系，如果某方持有另一方的集合，则使用< collection >标签完成映射，如果某方持有另一方的对象，则使用< association >标签完成映射。