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。
1 public class Minister { 2 private Integer mid; 3 private String mname; 4 5 public Integer getMid() { 6 return mid; 7 } 8 9 public void setMid(Integer mid) { 10 this.mid = mid; 11 } 12 13 public String getMname() { 14 return mname; 15 } 16 17 public void setMname(String mname) { 18 this.mname = mname; 19 } 20 21 @Override 22 public String toString() { 23 return "Minister [mid=" + mid + ", mname=" + mname + "]"; 24 } 25 26 }
1 import java.util.Set; 2 3 public class Country { 4 private Integer cid; 5 private String cname; 6 // 关联属性 7 private Set<Minister> ministers;// 一般用set 8 9 public Integer getCid() { 10 return cid; 11 } 12 13 public void setCid(Integer cid) { 14 this.cid = cid; 15 } 16 17 public String getCname() { 18 return cname; 19 } 20 21 public void setCname(String cname) { 22 this.cname = cname; 23 } 24 25 public Set<Minister> getMinisters() { 26 return ministers; 27 } 28 29 public void setMinisters(Set<Minister> ministers) { 30 this.ministers = ministers; 31 } 32 33 @Override 34 public String toString() { 35 return "Country [cid=" + cid + ", cname=" + cname + ", ministers=" + ministers + "]"; 36 } 37 38 }
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); } }
1 public interface ICountryDao { 2 Country selectCountryById(int cid); 3 }
对应实体类，Mapper.xml中
1 <mapper namespace="com.jmu.dao.ICountryDao"> 2 <resultMap type="Country" id="countryMapper"> 3 <id column="cid" property="cid" /> 4 <result column="cname" property="cname" /> 5 <collection property="ministers" ofType="Minister"><!-- ofType="Minister",要封装的类，也是集合的泛型 --> 6 <id column="mid" property="mid" /> 7 <result column="mname" property="mname" /> 8 </collection> 9 </resultMap> 10 <select id="selectCountryById" resultMap="countryMapper"> 11 select cid,cname,mid ,mname 12 from country,minister 13 where countryId=cid and cid=#{xxx} 14 </select> 15 </mapper>
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]]]
2、通过多表单独查询方式实现
1 <mapper namespace="com.jmu.dao.ICountryDao"> 2 <select id="selectMinisterByCountry" resultType="Minister"> 3 select mid,mname from minister where countryId=#{ooo} 4 </select> 5 <resultMap type="Country" id="countryMapper"> 6 <id column="cid" property="cid" /> 7 <result column="cname" property="cname" /> 8 <collection property="ministers" ofType="Minister" 9 select="selectMinisterByCountry" column="cid" /><!-- ofType="Minister",要封装的类，也是集合的泛型 --> 10 </resultMap> 11 <select id="selectCountryById" resultMap="countryMapper"> 12 select cid,cname from 13 country where cid=#{xxx} 14 </select> 15 </mapper>
1 0 [main] DEBUG com.jmu.dao.ICountryDao.selectCountryById - ==> Preparing: select cid,cname from country where cid=? 2 45 [main] DEBUG com.jmu.dao.ICountryDao.selectCountryById - ==> Parameters: 2(Integer) 3 79 [main] TRACE com.jmu.dao.ICountryDao.selectCountryById - <== Columns: cid, cname 4 80 [main] TRACE com.jmu.dao.ICountryDao.selectCountryById - <== Row: 2, England 5 84 [main] DEBUG com.jmu.dao.ICountryDao.selectMinisterByCountry - ====> Preparing: select mid,mname from minister where countryId=? 6 85 [main] DEBUG com.jmu.dao.ICountryDao.selectMinisterByCountry - ====> Parameters: 2(Integer) 7 91 [main] TRACE com.jmu.dao.ICountryDao.selectMinisterByCountry - <==== Columns: mid, mname 8 92 [main] TRACE com.jmu.dao.ICountryDao.selectMinisterByCountry - <==== Row: 4, ddd 9 93 [main] TRACE com.jmu.dao.ICountryDao.selectMinisterByCountry - <==== Row: 5, eee 10 94 [main] DEBUG com.jmu.dao.ICountryDao.selectMinisterByCountry - <==== Total: 2 11 95 [main] DEBUG com.jmu.dao.ICountryDao.selectCountryById - <== Total: 1 12 Country [cid=2, cname=England, ministers=[Minister [mid=4, mname=ddd], Minister [mid=5, mname=eee]]]
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
1 //新闻栏目：当前的新闻栏目被看作是一方，即父栏目 2 3 import java.util.Set; 4 5 public class NewLabel{ 6 private Integer id; 7 private String name;// 栏目名称 8 private Set<NewLabel> children; 9 10 public Integer getId() { 11 return id; 12 } 13 14 public void setId(Integer id) { 15 this.id = id; 16 } 17 18 public String getName() { 19 return name; 20 } 21 22 public void setName(String name) { 23 this.name = name; 24 } 25 26 public Set<NewLabel> getChildren() { 27 return children; 28 } 29 30 public void setChildren(Set<NewLabel> children) { 31 this.children = children; 32 } 33 34 @Override 35 public String toString() { 36 return "NewLable [id=" + id + ", name=" + name + ", children=" + children + "]"; 37 } 38 39 }
（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
1 import com.jmu.bean.NewsLabel; 2 3 public interface INewsLabelDao { 4 List<NewsLabel> selectChildrenByParent(int pid) ; 5 }
1 @Test 2 public void test01() { 3 List<NewsLabel> children=dao.selectChildrenByParent(2); 4 for (NewsLabel newLabel : children) { 5 System.out.println(newLabel); 6 } 7 }
1 <mapper namespace="com.jmu.dao.INewsLabelDao"> 2 <!-- <select id="selectChildrenByParent" resultMap="newslabelMapper"> 3 select id,name from newslabel where pid=#{ooo} 4 </select> --> 5 <resultMap type="NewsLabel" id="newslabelMapper"> 6 <id column="id" property="id"/> 7 <result column="name" property="name"/> 8 <collection property="children" 9 ofType="NewsLabel" 10 select="selectChildrenByParent" 11 column="id" 12 ></collection> 13 </resultMap> 14 <select id="selectChildrenByParent" resultMap="newslabelMapper"> 15 select id,name from newslabel where pid=#{xxx} 16 </select> 17 </mapper>
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=[]]]]
- 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。
1 import com.jmu.bean.NewsLabel; 2 3 public interface INewsLabelDao { 4 NewsLabel selectNewsLabelById(int id); 5 }
1 @Test 2 public void test01() { 3 NewsLabel newsLabel=dao.selectNewsLabelById(2); 4 System.out.println(newsLabel); 5 }
1 <mapper namespace="com.jmu.dao.INewsLabelDao"> 2 <select id="selectNewslabelByParent" resultMap="newslabelMapper"> 3 select id,name from newslabel where pid=#{ooo} 4 </select> 5 <resultMap type="NewsLabel" id="newslabelMapper"> 6 <id column="id" property="id"/> 7 <result column="name" property="name"/> 8 <collection property="children" 9 ofType="NewsLabel" 10 select="selectNewslabelByParent" 11 column="id" 12 ></collection> 13 </resultMap> 14 <select id="selectNewsLabelById" resultMap="newslabelMapper"> 15 select id,name from newslabel where id=#{xxx} 16 </select> 17 </mapper>
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=[]]]]]]
（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
1 //新闻栏目：当前的新闻栏目被看作是多方，即子栏目 2 public class NewsLabel{ 3 private Integer id; 4 private String name;// 栏目名称 5 private NewsLabel parent;//父栏目 6 // private Set<NewsLabel> children;//子栏目，完整自关联 7 public Integer getId() { 8 return id; 9 } 10 11 public void setId(Integer id) { 12 this.id = id; 13 } 14 15 public String getName() { 16 return name; 17 } 18 19 public void setName(String name) { 20 this.name = name; 21 } 22 23 public NewsLabel getParent() { 24 return parent; 25 } 26 27 public void setParent(NewsLabel parent) { 28 this.parent = parent; 29 } 30 31 @Override 32 public String toString() { 33 return "NewsLabel [id=" + id + ", name=" + name + ", parent=" + parent + "]"; 34 } 35 36 37 38 }
1 public interface INewsLabelDao { 2 NewsLabel selectNewsLabelById(int id); 3 }
1 public void test01() { 2 NewsLabel newsLabel=dao.selectNewsLabelById(3); 3 System.out.println(newsLabel); 4 }
1 <mapper namespace="com.jmu.dao.INewsLabelDao"> 2 <!-- <select id="selectNewslabelByParent" resultMap="newslabelMapper"> 3 select id,name,pid from newslabel where id=#{ooo} 4 </select> --> 5 <resultMap type="NewsLabel" id="newslabelMapper"> 6 <id column="id" property="id"/> 7 <result column="name" property="name"/> 8 <association property="parent" 9 javaType="NewsLabel" 10 select="selectNewsLabelById" 11 column="pid"></association> 12 </resultMap> 13 <select id="selectNewsLabelById" resultMap="newslabelMapper"> 14 select id,name,pid from newslabel where id=#{xxx} 15 </select> 16 </mapper>
输出：
1 0 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById - ==> Preparing: select id,name,pid from newslabel where id=? 2 57 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById - ==> Parameters: 3(Integer) 3 104 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewsLabelById - <== Columns: id, name, pid 4 108 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewsLabelById - <== Row: 3, NBA, 2 5 111 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById - ====> Preparing: select id,name,pid from newslabel where id=? 6 112 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById - ====> Parameters: 2(Integer) 7 112 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewsLabelById - <==== Columns: id, name, pid 8 113 [main] TRACE com.jmu.dao.INewsLabelDao.selectNewsLabelById - <==== Row: 2, 体育新闻, 0 9 113 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById - ======> Preparing: select id,name,pid from newslabel where id=? 10 113 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById - ======> Parameters: 0(Integer) 11 114 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById - <====== Total: 0 12 115 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById - <==== Total: 1 13 115 [main] DEBUG com.jmu.dao.INewsLabelDao.selectNewsLabelById - <== Total: 1 14 NewsLabel [id=3, name=NBA, parent=NewsLabel [id=2, name=体育新闻, parent=null]]
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
1 import java.util.Set; 2 3 public class Course { 4 private Integer cid; 5 private String cname; 6 private Set<Student> students; 7 8 public Integer getCid() { 9 return cid; 10 } 11 12 public void setCid(Integer cid) { 13 this.cid = cid; 14 } 15 16 public String getCname() { 17 return cname; 18 } 19 20 public void setCname(String cname) { 21 this.cname = cname; 22 } 23 24 public Set<Student> getStudents() { 25 return students; 26 } 27 28 public void setStudents(Set<Student> students) { 29 this.students = students; 30 } 31 32 @Override 33 public String toString() { 34 return "Course [cid=" + cid + ", cname=" + cname + ", students=" + students + "]"; 35 } 36 37 }
1 import java.util.Set; 2 3 public class Student { 4 private Integer sid; 5 private String sname; 6 private Set<Course> courses; 7 8 public Integer getSid() { 9 return sid; 10 } 11 12 public void setSid(Integer sid) { 13 this.sid = sid; 14 } 15 16 public String getSname() { 17 return sname; 18 } 19 20 public void setSname(String sname) { 21 this.sname = sname; 22 } 23 24 public Set<Course> getCourses() { 25 return courses; 26 } 27 28 public void setCourses(Set<Course> courses) { 29 this.courses = courses; 30 } 31 32 @Override 33 public String toString() { 34 return "Student [sid=" + sid + ", sname=" + sname + ", courses=" + courses + "]"; 35 } 36 37 }
1 import com.jmu.bean.Student; 2 3 public interface IStudentDao { 4 Student selectStudentById(int sid); 5 }
1 @Test 2 public void test01() { 3 Student student = dao.selectStudentById(1); 4 System.out.println(student); 5 }
1 <mapper namespace="com.jmu.dao.IStudentDao"> 2 <resultMap type="Student" id="studentMapper"> 3 <id column="sid" property="sid" /> 4 <result column="sname" property="sname" /> 5 <collection property="courses" ofType="Course"> 6 <id column="cid" property="cid" /> 7 <result column="cname" property="cname" /> 8 </collection> 9 </resultMap> 10 <select id="selectStudentById" resultMap="studentMapper"> 11 select sid,sname,cid,cname 12 from student1,middle,course 13 where sid=studentId and cid=courseId and sid=#{xxx} 14 </select> 15 </mapper>
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]]]
