Mybatis实现一对一查询和一对多查询

Mybatis中实现一对一多表联合查询与一对多多表联合查询。

举例：以用户和商品订单为例：

一个商品订单应该对应一个用户，而一个用户可以有多个商品订单，此时需要在用户的实体类中添加商品订单的List，同时在商品订单中添加用户User，实体类创建如下：

User类：

package com.w.pojo;

import java.io.Serializable;
import java.util.Date;
import java.util.List;

public class User implements Serializable {
	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;
	private Integer id;
	private String username;// 用户姓名
	private String sex;// 性别
	private Date birthday;// 生日
	private String address;// 地址
	private List<Orders> orders;
	


	public List<Orders> getOrders() {
		return orders;
	}
	public void setOrders(List<Orders> orders) {
		this.orders = orders;
	}
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getUsername() {
		return username;
	}
	public void setUsername(String username) {
		this.username = username;
	}
	public String getSex() {
		return sex;
	}
	public void setSex(String sex) {
		this.sex = sex;
	}
	public Date getBirthday() {
		return birthday;
	}
	public void setBirthday(Date birthday) {
		this.birthday = birthday;
	}
	public String getAddress() {
		return address;
	}
	public void setAddress(String address) {
		this.address = address;
	}
	@Override
	public String toString() {
		return "User [id=" + id + ", username=" + username + ", sex=" + sex
				+ ", birthday=" + birthday + ", address=" + address + "]";
	}

	
	

}

商品类

package com.w.pojo;

import java.io.Serializable;
import java.util.Date;

public class Orders  implements Serializable{
    @Override
	public String toString() {
		return "Orders [id=" + id + ", userId=" + userId + ", number=" + number + ", createtime=" + createtime
				+ ", note=" + note + "]";
	}

	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;

	private Integer id;

    private Integer userId;

    private String number;

    private Date createtime;

    private String note;
    
    private User user;
    

    public User getUser() {
		return user;
	}

	public void setUser(User user) {
		this.user = user;
	}

	public Integer getId() {
        return id;
    }

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

    public Integer getUserId() {
        return userId;
    }

    public void setUserId(Integer userId) {
        this.userId = userId;
    }

    public String getNumber() {
        return number;
    }

    public void setNumber(String number) {
        this.number = number == null ? null : number.trim();
    }

    public Date getCreatetime() {
        return createtime;
    }

    public void setCreatetime(Date createtime) {
        this.createtime = createtime;
    }

    public String getNote() {
        return note;
    }

    public void setNote(String note) {
        this.note = note == null ? null : note.trim();
    }

    
    
}

对应的xml文件应该这么添加：以用户为主表去查询用户列表，每个用户对应多个订单，所以订单为一个集合，具体添加如下：

<resultMap type="User" id="listuser">
		<id column="user_id" property="id"/>
		<result column="username" property="username"/>
		<collection property="orders" ofType="Orders">
		<id column="id" property="id"/>
		<result column="number" property="number"/>
		<result column="createtime" property="createtime"/>
		</collection>
	</resultMap>
	<select id="getUserLists" resultMap="listuser">
		SELECT
		o.id,
		o.user_id,
		o.number,
		o.createtime,
		u.username
		FROM
		user u
		LEFT JOIN
		orders o
		ON o.user_id = u.id
	</select>

注意：在集合中的collection的属性对应实体类中Orders中的属性，后面的list应该使用ofType="Orders"，同时在resultMap中应该将所有的列都写进去，以便对应数据库中对应的字段。

而以订单为主表去查询订单列表，每个订单包含一个用户实体类，其对应的mapper应该如下：

<resultMap type="Orders" id="orderlist">
		<id column="id" property="id" />
		<result column="number" property="number" />
		<result column="createtime" property="createtime" />
		<association property="user" javaType="User">
			<id column="user_id" property="id" />
			<result column="username" property="username" />
		</association>
	</resultMap>
	<select id="getOrderList" resultMap="orderlist">
		SELECT
		o.id,
		o.user_id,
		o.number,
		o.createtime,
		u.username
		FROM
		orders o
		LEFT JOIN
		user u
		on
		o.user_id=u.id
	</select>

而数据库的设计如下：

这样就可以实现一对一以及一对多的查询了，希望对你有帮助。喜欢的朋友可以关注我微信的公众号：从小白到架构师。我会每天更新学习内容的。