MyBatis_关联关系查询

一、关联查询

当查询的内容涉及到具有关联关系的多个表时，就需要使用关联查询。根据表与表间的关联关系的不同。关联查询分为四种：

1. 一对一关联查询；
2. 一对多关联查询；
3. 多对一关联查询；
4. 多对多关联查询；

二、一对多关联查询

1、通过多表连接查询方式实现

    定义实体类及DB表

一对一关联查询，其解决方案与多对一解决方案是相同的。

一对多关联查询：在查询一方对象的时候，同时将其所关联的多方对象也都查询出来。

Demo中使用国家（country）和部长（minister）进行示例。

 public class Minister {
     private Integer mid;
     private String mname;

     public Integer getMid() {
         return mid;
     }

     public void setMid(Integer mid) {
         this.mid = mid;
     }

     public String getMname() {
         return mname;
     }

     public void setMname(String mname) {
         this.mname = mname;
     }

     @Override
     public String toString() {
         return "Minister [mid=" + mid + ", mname=" + mname + "]";
     }

 }

Minister

 import java.util.Set;

 public class Country {
     private Integer cid;
     private String cname;
     // 关联属性
     private Set<Minister> ministers;// 一般用set

     public Integer getCid() {
         return cid;
     }

     public void setCid(Integer cid) {
         this.cid = cid;
     }

     public String getCname() {
         return cname;
     }

     public void setCname(String cname) {
         this.cname = cname;
     }

     public Set<Minister> getMinisters() {
         return ministers;
     }

     public void setMinisters(Set<Minister> ministers) {
         this.ministers = ministers;
     }

     @Override
     public String toString() {
         return "Country [cid=" + cid + ", cname=" + cname + ", ministers=" + ministers + "]";
     }

 }

Country

这里关联属性，一般一对多，用Set。(Set不能重复，list和array)。

数据库创建contry表和minister表

import org.apache.ibatis.session.SqlSession;
import org.apache.log4j.BasicConfigurator;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import com.jmu.bean.Country;
import com.jmu.dao.ICountryDao;
import com.jmu.utils.MybatisUtils;

public class MyTest {
    private ICountryDao dao;
    private SqlSession sqlSession;

    @Before
    public void Before() {
        sqlSession = MybatisUtils.getSqlSession();
        dao = sqlSession.getMapper(ICountryDao.class);
        BasicConfigurator.configure();
    }
  @After
  public void after(){
      if (sqlSession!=null) {
          sqlSession.commit();

    }

  }

    @Test
    public void test01() {
        Country country = dao.selectCountryById(2);
        System.out.println(country);
    }

}

com.jmu.test.MyTest

 public interface ICountryDao {
   Country selectCountryById(int cid);
 }

com.jmu.dao.ICountryDao

对应实体类，Mapper.xml中

 <mapper namespace="com.jmu.dao.ICountryDao">
     <resultMap type="Country" id="countryMapper">
         <id column="cid" property="cid" />
         <result column="cname" property="cname" />
         <collection property="ministers" ofType="Minister"><!-- ofType="Minister",要封装的类，也是集合的泛型 -->
             <id column="mid" property="mid" />
             <result column="mname" property="mname" />
         </collection>
     </resultMap>
     <select id="selectCountryById" resultMap="countryMapper">
         select cid,cname,mid ,mname
         from country,minister
         where countryId=cid and cid=#{xxx}
     </select>
 </mapper>

/mybatis8-one2many/src/com/jmu/dao/mapper.xml

输出：

0 [main] DEBUG com.jmu.dao.ICountryDao.selectCountryById  - ==>  Preparing: select cid,cname,mid ,mname from country,minister where countryId=cid and cid=?
57 [main] DEBUG com.jmu.dao.ICountryDao.selectCountryById  - ==> Parameters: 2(Integer)
96 [main] TRACE com.jmu.dao.ICountryDao.selectCountryById  - <==    Columns: cid, cname, mid, mname
96 [main] TRACE com.jmu.dao.ICountryDao.selectCountryById  - <==        Row: 2, England, 4, ddd
99 [main] TRACE com.jmu.dao.ICountryDao.selectCountryById  - <==        Row: 2, England, 5, eee
99 [main] DEBUG com.jmu.dao.ICountryDao.selectCountryById  - <==      Total: 2
Country [cid=2, cname=England, ministers=[Minister [mid=4, mname=ddd], Minister [mid=5, mname=eee]]]

output

2、通过多表单独查询方式实现

 <mapper namespace="com.jmu.dao.ICountryDao">
     <select id="selectMinisterByCountry" resultType="Minister">
         select mid,mname from minister where countryId=#{ooo}
     </select>
     <resultMap type="Country" id="countryMapper">
         <id column="cid" property="cid" />
         <result column="cname" property="cname" />
         <collection property="ministers" ofType="Minister"
             select="selectMinisterByCountry" column="cid" /><!-- ofType="Minister",要封装的类，也是集合的泛型 -->
     </resultMap>
     <select id="selectCountryById" resultMap="countryMapper">
         select cid,cname from
         country where cid=#{xxx}
     </select>
 </mapper>

/mybatis8-one2many2-2/src/com/jmu/dao/mapper.xml

 0 [main] DEBUG com.jmu.dao.ICountryDao.selectCountryById  - ==>  Preparing: select cid,cname from country where cid=?
 45 [main] DEBUG com.jmu.dao.ICountryDao.selectCountryById  - ==> Parameters: 2(Integer)
 79 [main] TRACE com.jmu.dao.ICountryDao.selectCountryById  - <==    Columns: cid, cname
 80 [main] TRACE com.jmu.dao.ICountryDao.selectCountryById  - <==        Row: 2, England
 84 [main] DEBUG com.jmu.dao.ICountryDao.selectMinisterByCountry  - ====>  Preparing: select mid,mname from minister where countryId=?
 85 [main] DEBUG com.jmu.dao.ICountryDao.selectMinisterByCountry  - ====> Parameters: 2(Integer)
 91 [main] TRACE com.jmu.dao.ICountryDao.selectMinisterByCountry  - <====    Columns: mid, mname
 92 [main] TRACE com.jmu.dao.ICountryDao.selectMinisterByCountry  - <====        Row: 4, ddd
 93 [main] TRACE com.jmu.dao.ICountryDao.selectMinisterByCountry  - <====        Row: 5, eee
 94 [main] DEBUG com.jmu.dao.ICountryDao.selectMinisterByCountry  - <====      Total: 2
 95 [main] DEBUG com.jmu.dao.ICountryDao.selectCountryById  - <==      Total: 1
 Country [cid=2, cname=England, ministers=[Minister [mid=4, mname=ddd], Minister [mid=5, mname=eee]]]

output

3、自关联查询

所谓自关联是指，自己既充当一方，又充当多方，是1：n或n：1的变型。

下面以新闻栏目为例

（1）自关联的DB表

 //新闻栏目：当前的新闻栏目被看作是一方，即父栏目

 import java.util.Set;

 public class NewLabel{
     private Integer id;
     private String name;// 栏目名称
     private Set<NewLabel> children;

     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 Set<NewLabel> getChildren() {
         return children;
     }

     public void setChildren(Set<NewLabel> children) {
         this.children = children;
     }

     @Override
     public String toString() {
         return "NewLable [id=" + id + ", name=" + name + ", children=" + children + "]";
     }

 }

com.jmu.bean.NewLabel

（2）以一对多方式处理

• 查询指定栏目的所有子孙栏目

新闻栏目：当前的新闻栏目被看作是一方，即父栏目

 import com.jmu.bean.NewsLabel;

 public interface INewsLabelDao {
    List<NewsLabel> selectChildrenByParent(int pid) ;
 }

com.jmu.dao.INewsLabelDao

 @Test
     public void test01() {
      List<NewsLabel> children=dao.selectChildrenByParent(2);
      for (NewsLabel newLabel : children) {
         System.out.println(newLabel);
     }
     }

MyTest

 <mapper namespace="com.jmu.dao.INewsLabelDao">
     <!-- <select id="selectChildrenByParent" resultMap="newslabelMapper">
        select id,name from newslabel where pid=#{ooo}
     </select> -->
     <resultMap type="NewsLabel" id="newslabelMapper">
       <id column="id" property="id"/>
       <result column="name" property="name"/>
       <collection property="children"
                   ofType="NewsLabel"
                   select="selectChildrenByParent"
                   column="id"
       ></collection>
     </resultMap>
     <select id="selectChildrenByParent" resultMap="newslabelMapper">
       select id,name from newslabel where pid=#{xxx}
     </select>
 </mapper>

/mybatis9-oneself-one2many/src/com/jmu/dao/mapper.xml

输出：

0 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ==>  Preparing: select id,name from newslabel where pid=?
47 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ==> Parameters: 2(Integer)
83 [main] TRACE com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <==    Columns: id, name
83 [main] TRACE com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <==        Row: 3, NBA
87 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ====>  Preparing: select id,name from newslabel where pid=?
88 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ====> Parameters: 3(Integer)
89 [main] TRACE com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <====    Columns: id, name
89 [main] TRACE com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <====        Row: 5, 火箭
89 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ======>  Preparing: select id,name from newslabel where pid=?
89 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ======> Parameters: 5(Integer)
90 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <======      Total: 0
91 [main] TRACE com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <====        Row: 6, 湖人
92 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ======>  Preparing: select id,name from newslabel where pid=?
92 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ======> Parameters: 6(Integer)
93 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <======      Total: 0
93 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <====      Total: 2
94 [main] TRACE com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <==        Row: 4, CBA
95 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ====>  Preparing: select id,name from newslabel where pid=?
95 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ====> Parameters: 4(Integer)
96 [main] TRACE com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <====    Columns: id, name
96 [main] TRACE com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <====        Row: 7, 北京金隅
97 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ======>  Preparing: select id,name from newslabel where pid=?
97 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ======> Parameters: 7(Integer)
98 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <======      Total: 0
98 [main] TRACE com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <====        Row: 8, 浙江广厦
98 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ======>  Preparing: select id,name from newslabel where pid=?
98 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ======> Parameters: 8(Integer)
99 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <======      Total: 0
99 [main] TRACE com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <====        Row: 9, 青岛双星
100 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ======>  Preparing: select id,name from newslabel where pid=?
101 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - ======> Parameters: 9(Integer)
102 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <======      Total: 0
102 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <====      Total: 3
102 [main] DEBUG com.jmu.dao.INewsLabelDao.selectChildrenByParent  - <==      Total: 2
NewLable [id=3, name=NBA, children=[NewLable [id=5, name=火箭, children=[]], NewLable [id=6, name=湖人, children=[]]]]
NewLable [id=4, name=CBA, children=[NewLable [id=9, name=青岛双星, children=[]], NewLable [id=7, name=北京金隅, children=[]], NewLable [id=8, name=浙江广厦, children=[]]]]

output

• 查询指定栏目及其所有子孙栏目

这里的查询结果，即要包含指定id的当前栏目，还包含其所有子孙栏目。

 import com.jmu.bean.NewsLabel;

 public interface INewsLabelDao {
   NewsLabel selectNewsLabelById(int id);
 }

com.jmu.dao.INewsLabelDao

 @Test
     public void test01() {
      NewsLabel newsLabel=dao.selectNewsLabelById(2);
      System.out.println(newsLabel);
     }

MyTest

 <mapper namespace="com.jmu.dao.INewsLabelDao">
     <select id="selectNewslabelByParent" resultMap="newslabelMapper">
       select id,name from newslabel where pid=#{ooo}
     </select>
     <resultMap type="NewsLabel" id="newslabelMapper">
       <id column="id" property="id"/>
       <result column="name" property="name"/>
       <collection property="children"
                   ofType="NewsLabel"
                   select="selectNewslabelByParent"
                   column="id"
       ></collection>
     </resultMap>
     <select id="selectNewsLabelById" resultMap="newslabelMapper">
       select id,name from newslabel where id=#{xxx}
     </select>
 </mapper>

mapper.xml

输出：

0 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - ==>  Preparing: select id,name from newslabel where id=?
48 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - ==> Parameters: 2(Integer)
96 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewsLabelById  - <==    Columns: id, name
97 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewsLabelById  - <==        Row: 2, 体育新闻
101 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ====>  Preparing: select id,name from newslabel where pid=?
105 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ====> Parameters: 2(Integer)
106 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <====    Columns: id, name
106 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <====        Row: 3, NBA
107 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ======>  Preparing: select id,name from newslabel where pid=?
107 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ======> Parameters: 3(Integer)
108 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <======    Columns: id, name
108 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <======        Row: 5, 火箭
109 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ========>  Preparing: select id,name from newslabel where pid=?
109 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ========> Parameters: 5(Integer)
110 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <========      Total: 0
111 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <======        Row: 6, 湖人
113 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ========>  Preparing: select id,name from newslabel where pid=?
114 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ========> Parameters: 6(Integer)
115 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <========      Total: 0
115 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <======      Total: 2
115 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <====        Row: 4, CBA
116 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ======>  Preparing: select id,name from newslabel where pid=?
116 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ======> Parameters: 4(Integer)
117 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <======    Columns: id, name
117 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <======        Row: 7, 北京金隅
117 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ========>  Preparing: select id,name from newslabel where pid=?
118 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ========> Parameters: 7(Integer)
118 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <========      Total: 0
119 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <======        Row: 8, 浙江广厦
119 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ========>  Preparing: select id,name from newslabel where pid=?
120 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ========> Parameters: 8(Integer)
121 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <========      Total: 0
121 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <======        Row: 9, 青岛双星
123 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ========>  Preparing: select id,name from newslabel where pid=?
124 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - ========> Parameters: 9(Integer)
125 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <========      Total: 0
125 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <======      Total: 3
126 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewslabelByParent  - <====      Total: 2
126 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - <==      Total: 1
NewLable [id=2, name=体育新闻, children=[NewLable [id=4, name=CBA, children=[NewLable [id=7, name=北京金隅, children=[]], NewLable [id=9, name=青岛双星, children=[]], NewLable [id=8, name=浙江广厦, children=[]]]], NewLable [id=3, name=NBA, children=[NewLable [id=5, name=火箭, children=[]], NewLable [id=6, name=湖人, children=[]]]]]]

output

（3）以多对一方式实现

• 查询当前栏目及其所有父辈栏目

新闻栏目：当前的新闻栏目被看作是多方，即子栏目

 //新闻栏目：当前的新闻栏目被看作是多方，即子栏目
 public class NewsLabel{
     private Integer id;
     private String name;// 栏目名称
     private NewsLabel parent;//父栏目
   //  private Set<NewsLabel> children;//子栏目，完整自关联
     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 NewsLabel getParent() {
         return parent;
     }

     public void setParent(NewsLabel parent) {
         this.parent = parent;
     }

     @Override
     public String toString() {
         return "NewsLabel [id=" + id + ", name=" + name + ", parent=" + parent + "]";
     }

 }

com.jmu.bean.NewsLabel

 public interface INewsLabelDao {
   NewsLabel selectNewsLabelById(int id);
 }

com.jmu.dao.INewsLabelDao

 public void test01() {
      NewsLabel newsLabel=dao.selectNewsLabelById(3);
      System.out.println(newsLabel);
     }

MyTest

 <mapper namespace="com.jmu.dao.INewsLabelDao">
     <!-- <select id="selectNewslabelByParent" resultMap="newslabelMapper">
       select id,name,pid  from newslabel where id=#{ooo}
     </select> -->
     <resultMap type="NewsLabel" id="newslabelMapper">
       <id column="id" property="id"/>
       <result column="name" property="name"/>
       <association property="parent"
                    javaType="NewsLabel"
                    select="selectNewsLabelById"
                    column="pid"></association>
     </resultMap>
     <select id="selectNewsLabelById" resultMap="newslabelMapper">
       select id,name,pid from newslabel where id=#{xxx}
     </select>
 </mapper>

mapper.xml

输出：

 0 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - ==>  Preparing: select id,name,pid from newslabel where id=?
 57 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - ==> Parameters: 3(Integer)
 104 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewsLabelById  - <==    Columns: id, name, pid
 108 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewsLabelById  - <==        Row: 3, NBA, 2
 111 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - ====>  Preparing: select id,name,pid from newslabel where id=?
 112 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - ====> Parameters: 2(Integer)
 112 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewsLabelById  - <====    Columns: id, name, pid
 113 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewsLabelById  - <====        Row: 2, 体育新闻, 0
 113 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - ======>  Preparing: select id,name,pid from newslabel where id=?
 113 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - ======> Parameters: 0(Integer)
 114 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - <======      Total: 0
 115 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - <====      Total: 1
 115 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById  - <==      Total: 1
 NewsLabel [id=3, name=NBA, parent=NewsLabel [id=2, name=体育新闻, parent=null]]

output

4、多对多关联查询

例如：一个学生可以选多门课程，而一门课程可以由多门学生选。

一般情况下，多对多关心都会通过一个中间表来建立。例如选课表。

创建DB表和实体类：

middle表中有外键，为多方，即中间表为多方

　　

 import java.util.Set;

 public class Course {
     private Integer cid;
     private String cname;
     private Set<Student> students;

     public Integer getCid() {
         return cid;
     }

     public void setCid(Integer cid) {
         this.cid = cid;
     }

     public String getCname() {
         return cname;
     }

     public void setCname(String cname) {
         this.cname = cname;
     }

     public Set<Student> getStudents() {
         return students;
     }

     public void setStudents(Set<Student> students) {
         this.students = students;
     }

     @Override
     public String toString() {
         return "Course [cid=" + cid + ", cname=" + cname + ", students=" + students + "]";
     }

 }

com.jmu.bean.Course

 import java.util.Set;

 public class Student {
     private Integer sid;
     private String sname;
     private Set<Course> courses;

     public Integer getSid() {
         return sid;
     }

     public void setSid(Integer sid) {
         this.sid = sid;
     }

     public String getSname() {
         return sname;
     }

     public void setSname(String sname) {
         this.sname = sname;
     }

     public Set<Course> getCourses() {
         return courses;
     }

     public void setCourses(Set<Course> courses) {
         this.courses = courses;
     }

     @Override
     public String toString() {
         return "Student [sid=" + sid + ", sname=" + sname + ", courses=" + courses + "]";
     }

 }

com.jmu.bean.Student

 import com.jmu.bean.Student;

 public interface IStudentDao {
   Student selectStudentById(int sid);
 }

com.jmu.dao.IStudentDao

 @Test
     public void test01() {
       Student student = dao.selectStudentById(1);
       System.out.println(student);
     }

MyTest

 <mapper namespace="com.jmu.dao.IStudentDao">
     <resultMap type="Student" id="studentMapper">
         <id column="sid" property="sid" />
         <result column="sname" property="sname" />
         <collection property="courses" ofType="Course">
             <id column="cid" property="cid" />
             <result column="cname" property="cname" />
         </collection>
     </resultMap>
     <select id="selectStudentById" resultMap="studentMapper">
         select sid,sname,cid,cname
         from student1,middle,course
         where sid=studentId and cid=courseId and sid=#{xxx}
     </select>
 </mapper>

mapper.xml

输出：

0 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentById  - ==>  Preparing: select sid,sname,cid,cname from student1,middle,course where sid=studentId and cid=courseId and sid=?
69 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentById  - ==> Parameters: 1(Integer)
101 [main] TRACE com.jmu.dao.IStudentDao.selectStudentById  - <==    Columns: sid, sname, cid, cname
102 [main] TRACE com.jmu.dao.IStudentDao.selectStudentById  - <==        Row: 1, 刘备, 1, JavaSE
112 [main] TRACE com.jmu.dao.IStudentDao.selectStudentById  - <==        Row: 1, 刘备, 2, JavaEE
112 [main] DEBUG com.jmu.dao.IStudentDao.selectStudentById  - <==      Total: 2
Student [sid=1, sname=刘备, courses=[Course [cid=1, cname=JavaSE, students=null], Course [cid=2, cname=JavaEE, students=null]]]

output