MyBatis-多表查询（一对一）

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1、多表查询（一对一）

1.1、使用多表查询前的准备

学生表（students）：

班级表（classes）：

1.2、需求分析（一对一）

查询所有学生信息，关联查询下班级信息。

注意：

因为一个学生信息只能在一个班级里，所以从查询学生信息出发关联查询班级信息为一对一查询。

如果从班级信息出发查询班级下的学生信息则为一对多查询，因为一个班级可以有多个学生。

方式一 SQL 如下：

select s.*,c.cname from students s,classes c where s.cid=c.cid;

方式二 SQL 如下：

select s.sid,s.sname,s.ssex,s.sage,c.* from students s,classes c where s.cid=c.cid

2、案例实现 - 方式一

2.1、编写实体类文件

Students.java

package com.tyschool.mb005.javabean;
 
import java.io.Serializable;
 
public class Students implements Serializable {private int sid;
    private String sname;
    private String ssex;
    private int sage;
 
    public int getSid() {return sid;
    }
 
    public void setSid(int sid) {this.sid = sid;
    }
 
    public String getSname() {return sname;
    }
 
    public void setSname(String sname) {this.sname = sname;
    }
 
    public String getSsex() {return ssex;
    }
 
    public void setSsex(String ssex) {this.ssex = ssex;
    }
 
    public int getSage() {return sage;
    }
 
    public void setSage(int sage) {this.sage = sage;
    }
 
    @Override
    public String toString() {return "Students{" +
                "sid=" + sid +
                ", sname='" + sname + '\'' +
                ", ssex='" + ssex + '\'' +
                ", sage=" + sage +
                '}';
    }
}

Classes.java

import java.io.Serializable;
 
public class Classes implements Serializable {private int cid;
    private String cname;
 
    public int getCid() {return cid;
    }
 
    public void setCid(int cid) {this.cid = cid;
    }
 
    public String getCname() {return cname;
    }
 
    public void setCname(String cname) {this.cname = cname;
    }
 
    @Override
    public String toString() {return "Classes{" +
                "cid=" + cid +
                ", cname='" + cname + '\'' +
                '}';
    }
}

2.2、定义返回值接收类文件

StudentsClasses.java

import java.io.Serializable;
 
public class StudentsClasses extends Students implements Serializable {private String cname;
 
    public String getCname() {return cname;
    }
 
    public void setCname(String cname) {this.cname = cname;
    }
 
    @Override
    public String toString() {return "StudentsClasses{" +
                "cname='" + cname + '\'' +
                '}';
    }
}

2.3、创建 IStudentsDao.java 文件

import com.tyschool.mb005.javabean.StudentsClasses;
 
import java.util.List;
 
public interface IStudentsDao {List<StudentsClasses> findAll();
}
 

2.4、创建 IStudentsDao.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.tyschool.mb005.students.dao.IStudentsDao">
<select id="findAll" resultType="com.tyschool.mb005.javabean.StudentsClasses">
        select s.*,c.cname from students s,classes c where s.cid=c.cid
    </select>
</mapper>

注意：

写好的 IStudentsDao.xml 文件要在 SqlMapConfig.xml 中加入映射

<mappers>
        <mapper resource="com/tyschool/mb005/user/dao/IStudentsDao.xml"></mapper>
</mappers>

2.5、创建测试类 MbStudentsTest.java 文件

package com.tyschool.mb005.test;
 
import com.tyschool.mb005.javabean.StudentsClasses;
import com.tyschool.mb005.students.dao.IStudentsDao;
import com.tyschool.mb005.user.dao.IUserDao;
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 org.junit.After;
import org.junit.Before;
import org.junit.Test;
 
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
 
public class MbStudentsTest {private InputStream in;
    private SqlSession session;
    private IStudentsDao studentsDao;
    @Test
    public void findAll(){List<StudentsClasses> list=studentsDao.findAll();
        for(StudentsClasses sc :list){System.out.println(sc);
        }
    }
    @Before
    public void init()throws IOException {in= Resources.getResourceAsStream("SqlMapConfig.xml");
        SqlSessionFactoryBuilder builder=new SqlSessionFactoryBuilder();
        SqlSessionFactory factory=builder.build(in);
        session=factory.openSession();
        studentsDao=session.getMapper(IStudentsDao.class);
    }
    @After
    public void destroy() throws IOException {session.commit();;
        session.close();
        in.close();}
}

3、案例实现 - 方式二

使用 resultMap，定义专门的 resultMap 用于映射一对一查询结果。
通过面向对象的关系可以得知，我们可以在 Students 类中加入一个 Classes 类的对象来代表这个学生是哪个班级的。

3.1、修改 Students.java 文件

加入 Classes 对象

private Classes classes;
 
public Classes getClasses() {return classes;
}
 
public void setClasses(Classes classes) {this.classes = classes;
}

3.2、修改 IStudentsDao.java 文件

import com.tyschool.mb005.javabean.Students;
import com.tyschool.mb005.javabean.StudentsClasses;
 
import java.util.List;
 
public interface IStudentsDao {List<StudentsClasses> findAll();
    List<Students> findAll1();
}

3.3、修改 IStudentsDao.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.tyschool.mb005.students.dao.IStudentsDao">
    <resultMap id="studentsMap" type="com.tyschool.mb005.javabean.Students">
        <id column="sid" property="sid"></id>
        <result column="sname" property="sname"></result>
        <result column="ssex" property="ssex"></result>
        <result column="sage" property="sage"></result>
        <association property="classes" javaType="com.tyschool.mb005.javabean.Classes">
            <id column="cid" property="cid"></id>
            <result column="cname" property="cname"></result>
        </association>
    </resultMap>
    <select id="findAll" resultType="com.tyschool.mb005.javabean.StudentsClasses">
        select s.*,c.cname from students s,classes c where s.cid=c.cid
    </select>
    <select id="findAll1" resultMap="studentsMap">
        select s.sid,s.sname,s.ssex,s.sage,c.* from students s,classes c where s.cid=c.cid
    </select>
</mapper>