• MyBatis支援多表操作，即可以將資料庫中多表的關係對映到物件之間的關係中
• 表與表之間的關係可以有：一對一，一對多，多對多
• 關係一演示案例：人和身份證是一對一的，分別建兩個表person和card，其中person的cardid欄位外來鍵關聯card的id，當需求是查詢某人姓名、年齡及身份證號時就需要外連線，查詢結果在MyBatis中是如何對映到物件中的呢？
• 備註：未用MyBatis框架前，我們通常將多表查詢的結果放在dto或vo包中的結果類裡進行封裝，有利有弊，程式碼多，而且碰到一對多的關係時會出現大量資料不可避免地重複封裝。 person表 card表 相應的兩個實體類

package com.yicha.entity;
//Card.java
public class Card {
    private int id;
    private String cardNum;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getCardNum() {
        return cardNum;
    }
    public void setCardNum
 
(String cardNum) {
        this.cardNum = cardNum;
    }

}

package com.yicha.entity;
//Person.java
public class Person {

    private int id;
    private String name;
    private int age;
    private Card card;

    public int getId() {
        return id;
    }
    public void setId(int id) {
        this
 
.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public Card getCard() {
        return card;
    }
    public void setCard(Card card) {
        this.card = card;
    }

}

Person.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//ibatis.apache.org//DTD SQL Map 3.0//EN"
"http://ibatis.apache.org/dtd/ibatis-3-mapper.dtd">
<mapper namespace="com.yicha.entity.Person">
    <!-- 對結果自定義對映方式 -->
    <resultMap type="Person" id="myMap">
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <!-- 一對一用association標籤 -->
        <!-- 這種對映方式由於是執行一條sql語句findStudent，無法懶載入（也就是按需載入） -->
        <association property="card" javaType="Card">
            <result property="cardNum" column="cardnum"/>
        </association>
    </resultMap>
    <!-- 對映結果為自定義Map -->
    <select id="findPerson" resultMap="myMap">
        SELECT a.name,a.age,b.cardnum 
        FROM person a
        LEFT JOIN card b
        ON a.cardid=b.id
        WHERE a.id=1
    </select>
</mapper>

SqlMapConfig.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//ibatis.apache.org//DTD SQL Map Config 3.0//EN"
"http://ibatis.apache.org/dtd/ibatis-3-config.dtd">
<configuration>
    <!-- 取別名 -->
    <typeAliases>
        <typeAlias type="com.yicha.entity.Person" alias="Person"/>
        <typeAlias type="com.yicha.entity.Card" alias="Card"/>
    </typeAliases>

    <!-- 配環境 -->
    <environments default="e1">
        <environment id="e1">
            <!-- 先配事務 -->
            <transactionManager type="JDBC"></transactionManager>
            <!-- 配資料來源 -->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/test"/>
                <property name="username" value="root"/>
                <property name="password" value="root1205"/>
            </dataSource>
        </environment>
    </environments>

    <!-- 實體類對映檔案 -->
    <mappers>
        <mapper resource="com/yicha/entity/Person.xml"/>
    </mappers>
</configuration>

測試類

package com.yicha.test;
//Demo1.java
import java.io.IOException;
import java.io.Reader;

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

public class Demo1 {

    public static void main(String[] args) {
        SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
        SqlSessionFactory factory = null;
        SqlSession session = null;
        try {
            Reader reader = Resources.getResourceAsReader("SqlMapConfig.xml");
            factory = builder.build(reader);
            session = factory.openSession();
        } catch (IOException e) {
            e.printStackTrace();
        }
        session.selectOne("findPerson");        
    }
}

結果