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：

一对一关联查询；
一对多关联查询；
多对一关联查询；
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

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