MyBatis 關聯查詢的實現：一對一

 

有2個實體：用戶、會員卡，一個用戶只能辦理一張會員卡，即一對一。

 

user_tb : 

 需要在一方引入另一方的主鍵作為外鍵。

 

 

card_tb:

 

 

---

 

 

使用擴展類

（1）在pojo包下新建User類：

package com.chy.pojo;

public class User {
    private Integer id;  //主鍵
    private String name;  //姓名
    private String tel;  //手機號
    private String address;  //地址

    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 String getTel() {
        return tel;
    }

    public void setTel(String tel) {
        this.tel = tel;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", tel='" + tel + '\'' +
                ", address='" + address + '\'' +
                '}';
    }
}

 

 

（2）在pojo包下新建User的擴展類UserExt，繼承User，並把Card的屬性添加進來，提供對應的getter、setter方法。

package com.chy.pojo;

public class UserExt extends User {
    private Integer no;
    private float money;

    public Integer getNo() {
        return no;
    }

    public void setNo(Integer no) {
        this.no = no;
    }

    public float getMoney() {
        return money;
    }

    public void setMoney(float money) {
        this.money = money;
    }

    @Override
    public String toString() {
        return super.toString()+",Card{" +
                "no=" + no +
                ", money=" + money +
                '}';
    }
}

先alt+insert插入toString()，再拼接上User的toString()，然后修改下就ok。

 

 

（3）編寫UserMapper接口、UserMapper.xml

package com.chy.mapper;

import com.chy.pojo.UserExt;

public interface UserMapper {
    public UserExt queryUserExtById(Integer id);
}

<?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.chy.mapper.UserMapper">
    <select id="queryUserExtById" parameterType="integer" resultType="userext">
        SELECT user_tb.*,card_tb.* FROM user_tb,card_tb WHERE user_tb.id=#{id} AND user_tb.card_no=card_tb.no
     </select>
</mapper>

 

 

(4)使用

package com.chy.utils;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import java.io.IOException;
import java.io.InputStream;

public class MyBatisUtils {
    private static SqlSessionFactory sqlSessionFactory;

    static {
        try {
            InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

 

     SqlSession sqlSession = MyBatisUtils.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        UserExt userExt = mapper.queryUserExtById(1);
        System.out.println(userExt);
        sqlSession.close();

 

使用擴展類可以實現一對一的關聯查詢，但沒有體現實體之間的關聯關系（一個模型中包含另一個模型）。

 

 

 

---

 

 

 

使用嵌套查詢

（1）給2個“一”都編寫pojo類，需要在一個“一”中關聯另一個“一”

package com.chy.pojo;

public class User {
    private Integer id;  //主鍵
    private String name;  //姓名
    private String tel;  //手機號
    private String address;  //地址
    private Card card;  //與之關聯的Card

    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 String getTel() {
        return tel;
    }

    public void setTel(String tel) {
        this.tel = tel;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public Card getCard() {
        return card;
    }

    public void setCard(Card card) {
        this.card = card;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", tel='" + tel + '\'' +
                ", address='" + address + '\'' +
                ", card=" + card +
                '}';
    }
}

 

package com.chy.pojo;

public class Card {
    private Integer no;  //會員卡編號
    private Float money;  //余額

    public Integer getNo() {
        return no;
    }

    public void setNo(Integer no) {
        this.no = no;
    }

    public Float getMoney() {
        return money;
    }

    public void setMoney(Float money) {
        this.money = money;
    }

    @Override
    public String toString() {
        return "Card{" +
                "no=" + no +
                ", money=" + money +
                '}';
    }
}

 外鍵是用來輔助sql操作的，並不是實體的屬性，所以pojo類一般不包含外鍵字段。

 

 

（2）給這2個pojo類都編寫Mapper接口、xml映射文件

public interface CardMapper {
    public Card queryCardByUserId(Integer no);
}

<?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.chy.mapper.CardMapper">
    <select id="queryCardByNo" parameterType="integer" resultType="card">
        SELECT * FROM card_tb WHERE no=#{no}
     </select>
</mapper>

 

 

package com.chy.mapper;

import com.chy.pojo.User;

public interface UserMapper {
    public User queryUserById(Integer id);
}

<?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.chy.mapper.UserMapper">
    <select id="queryUserById" parameterType="integer" resultMap="userResultWithCard">
        SELECT * FROM user_tb WHERE id=#{id}
     </select>
    <resultMap id="userResultWithCard" type="user">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="tel" column="tel"/>
        <result property="address" column="address"/>
        <association property="card" select="com.chy.mapper.CardMapper.queryCardByNo" column="card_no" javaType="card" /> 
　　 </resultMap>
</mapper>

sql語句都是查詢當前pojo類對應的數據表，但主動嵌套的查詢要使用resultMap來設置關聯對象的映射，被嵌套的查詢則不必設置。

• property指定表示關聯對象的成員變量名
• select指定要嵌套的查詢（namespace+id），執行當前查詢時，會自動嵌套指定的<select>進行查詢
• column指定要把當前pojo類對應的數據表的哪一列作為參數傳遞給select嵌套的查詢。
• javaType指定嵌套的查詢返回的數據類型

 

 

（3）使用

package com.chy.utils;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import java.io.IOException;
import java.io.InputStream;

public class MyBatisUtils {
    private static SqlSessionFactory sqlSessionFactory;

    static {
        try {
            InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

 

package com.chy.test;

import com.chy.mapper.UserMapper;
import com.chy.pojo.User;
import com.chy.utils.MyBatisUtils;
import org.apache.ibatis.session.*;
import java.io.IOException;

public class Test {
    public static void main(String[] args) {
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        User user = mapper.queryUserById(1);
        System.out.println(user);
        sqlSession.close();
    }
}

 

使用嵌套查詢體現了實體之間的關聯關系，但一條查詢會觸發另一個與之關聯的查詢，另一個查詢如果有與之關聯的查詢，也會觸發.....可能引發鏈式反應，降低查詢效率和數據庫的性能。

 

 

---

 

 

 

使用嵌套結果

使用方式與嵌套查詢大體相同，不同的只有第二步：

（二）編寫UserMapper接口、UserMapper.xml

package com.chy.mapper;

import com.chy.pojo.User;

public interface UserMapper {
    public User queryUserById(Integer id);
}

 

<?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.chy.mapper.UserMapper">
    <select id="queryUserById" parameterType="integer" resultMap="userResultWithCard">
        SELECT user_tb.*,card_tb.* FROM user_tb,card_tb WHERE user_tb.id=#{id} AND user_tb.card_no=card_tb.no
     </select>
    <resultMap id="userResultWithCard" type="user">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="tel" column="tel"/>
        <result property="address" column="address"/>
        <association property="card" javaType="card">
            <id property="no" column="no"/>
            <result property="money" column="money"/>
        </association>
    </resultMap>
</mapper>

因為只查詢一次，所以要選取所有需要的數據。

<association>指定與之關聯的實體的映射。

• property表示關聯對象的成員變量名
• javaType指定返回的數據類型，mybatis根據javaType找到關聯對象對應的pojo類。

嵌套結果只需指定2個屬性，而嵌套查詢要用select指定嵌套的<select>，還需要用column向嵌套的查詢傳遞參數，所以嵌套查詢需要指定4個屬性。