MyBatis学习总结（5）——实现关联表查询

 1 CREATE TABLE teacher(  2     t_id INT PRIMARY KEY AUTO_INCREMENT,  3     t_name VARCHAR(20)  4 );  5 CREATE TABLE class(  6     c_id INT PRIMARY KEY AUTO_INCREMENT,  7     c_name VARCHAR(20),  8     teacher_id INT
 9 ); 10 ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id); 11 
12 INSERT INTO teacher(t_name) VALUES('teacher1'); 13 INSERT INTO teacher(t_name) VALUES('teacher2'); 14 
15 INSERT INTO class(c_name, teacher_id) VALUES('class_a', 1); 16 INSERT INTO class(c_name, teacher_id) VALUES('class_b', 2);

 1 package me.gacl.domain;  2 
 3 /**
 4  * @author gacl  5  * 定义teacher表对应的实体类  6  */
 7 public class Teacher {  8 
 9     //定义实体类的属性，与teacher表中的字段对应
10     private int id;            //id===>t_id
11     private String name;    //name===>t_name
12 
13     public int getId() { 14         return id; 15  } 16 
17     public void setId(int id) { 18         this.id = id; 19  } 20 
21     public String getName() { 22         return name; 23  } 24 
25     public void setName(String name) { 26         this.name = name; 27  } 28 
29  @Override 30     public String toString() { 31         return "Teacher [id=" + id + ", name=" + name + "]"; 32  } 33 }

 1 package me.gacl.domain;  2 
 3 /**
 4  * @author gacl  5  * 定义class表对应的实体类  6  */
 7 public class Classes {  8 
 9     //定义实体类的属性，与class表中的字段对应
10     private int id;            //id===>c_id
11     private String name;    //name===>c_name
12     
13     /**
14  * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性， 15  * 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的 16      */
17     private Teacher teacher; 18 
19     public int getId() { 20         return id; 21  } 22 
23     public void setId(int id) { 24         this.id = id; 25  } 26 
27     public String getName() { 28         return name; 29  } 30 
31     public void setName(String name) { 32         this.name = name; 33  } 34 
35     public Teacher getTeacher() { 36         return teacher; 37  } 38 
39     public void setTeacher(Teacher teacher) { 40         this.teacher = teacher; 41  } 42 
43  @Override 44     public String toString() { 45         return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher+ "]"; 46  } 47 }

 1 <?xml version="1.0" encoding="UTF-8" ?>
 2 <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
 3 <!-- 为这个mapper指定一个唯一的namespace，namespace的值习惯上设置成包名+sql映射文件名，这样就能够保证namespace的值是唯一的  4 例如namespace="me.gacl.mapping.classMapper"就是me.gacl.mapping(包名)+classMapper(classMapper.xml文件去除后缀)  5  -->
 6 <mapper namespace="me.gacl.mapping.classMapper">
 7 
 8     <!-- 
 9  根据班级id查询班级信息(带老师的信息) 10  ##1. 联表查询 11  SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1; 12         
13  ##2. 执行两次查询 14  SELECT * FROM class WHERE c_id=1; //teacher_id=1 15  SELECT * FROM teacher WHERE t_id=1;//使用上面得到的teacher_id 16      -->
17 
18     <!-- 
19  方式一：嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集 20  封装联表查询的数据(去除重复的数据) 21  select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=1 22     -->
23     <select id="getClass" parameterType="int" resultMap="ClassResultMap">
24  select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id} 25     </select>
26     <!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
27     <resultMap type="me.gacl.domain.Classes" id="ClassResultMap">
28         <id property="id" column="c_id"/>
29         <result property="name" column="c_name"/>
30         <association property="teacher" javaType="me.gacl.domain.Teacher">
31             <id property="id" column="t_id"/>
32             <result property="name" column="t_name"/>
33         </association>
34     </resultMap>
35     
36     <!-- 
37  方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型 38  SELECT * FROM class WHERE c_id=1; 39  SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id的值 40     -->
41      <select id="getClass2" parameterType="int" resultMap="ClassResultMap2">
42  select * from class where c_id=#{id} 43      </select>
44      <!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
45      <resultMap type="me.gacl.domain.Classes" id="ClassResultMap2">
46         <id property="id" column="c_id"/>
47         <result property="name" column="c_name"/>
48         <association property="teacher" column="teacher_id" select="getTeacher"/>
49      </resultMap>
50      
51      <select id="getTeacher" parameterType="int" resultType="me.gacl.domain.Teacher">
52  SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id} 53      </select>
54 
55 </mapper>

<mappers>
        <!-- 注册classMapper.xml文件， classMapper.xml位于me.gacl.mapping这个包下，所以resource写成me/gacl/mapping/classMapper.xml-->
        <mapper resource="me/gacl/mapping/classMapper.xml"/>
</mappers>

 1 package me.gacl.test;  2 
 3 import me.gacl.domain.Classes;  4 import me.gacl.util.MyBatisUtil;  5 import org.apache.ibatis.session.SqlSession;  6 import org.junit.Test;  7 
 8 public class Test3 {  9     
10  @Test 11     public void testGetClass(){ 12         SqlSession sqlSession = MyBatisUtil.getSqlSession(); 13         /**
14  * 映射sql的标识字符串， 15  * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值， 16  * getClass是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL 17          */
18         String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的标识字符串 19         //执行查询操作，将查询结果自动封装成Classes对象返回
20         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录 21         //使用SqlSession执行完SQL之后需要关闭SqlSession
22  sqlSession.close(); 23         System.out.println(clazz);//打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
24  } 25     
26  @Test 27     public void testGetClass2(){ 28         SqlSession sqlSession = MyBatisUtil.getSqlSession(); 29         /**
30  * 映射sql的标识字符串， 31  * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值， 32  * getClass2是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL 33          */
34         String statement = "me.gacl.mapping.classMapper.getClass2";//映射sql的标识字符串 35         //执行查询操作，将查询结果自动封装成Classes对象返回
36         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录 37         //使用SqlSession执行完SQL之后需要关闭SqlSession
38  sqlSession.close(); 39         System.out.println(clazz);//打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
40  } 41 }

CREATE TABLE student( s_id INT PRIMARY KEY AUTO_INCREMENT, s_name VARCHAR(20), class_id INT ); INSERT INTO student(s_name, class_id) VALUES('student_A', 1); INSERT INTO student(s_name, class_id) VALUES('student_B', 1); INSERT INTO student(s_name, class_id) VALUES('student_C', 1); INSERT INTO student(s_name, class_id) VALUES('student_D', 2); INSERT INTO student(s_name, class_id) VALUES('student_E', 2); INSERT INTO student(s_name, class_id) VALUES('student_F', 2);

 1 package me.gacl.domain;  2 
 3 /**
 4  * @author gacl  5  * 定义student表所对应的实体类  6  */
 7 public class Student {  8 
 9     //定义属性，和student表中的字段对应
10     private int id;            //id===>s_id
11     private String name;    //name===>s_name
12     
13     public int getId() { 14         return id; 15  } 16 
17     public void setId(int id) { 18         this.id = id; 19  } 20 
21     public String getName() { 22         return name; 23  } 24 
25     public void setName(String name) { 26         this.name = name; 27  } 28 
29  @Override 30     public String toString() { 31         return "Student [id=" + id + ", name=" + name + "]"; 32  } 33 }

 1 package me.gacl.domain;  2 
 3 import java.util.List;  4 
 5 /**
 6  * @author gacl  7  * 定义class表对应的实体类  8  */
 9 public class Classes { 10 
11     //定义实体类的属性，与class表中的字段对应
12     private int id;            //id===>c_id
13     private String name;    //name===>c_name
14     
15     /**
16  * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性， 17  * 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的 18      */
19     private Teacher teacher; 20     //使用一个List<Student>集合属性表示班级拥有的学生
21     private List<Student> students; 22 
23     public int getId() { 24         return id; 25  } 26 
27     public void setId(int id) { 28         this.id = id; 29  } 30 
31     public String getName() { 32         return name; 33  } 34 
35     public void setName(String name) { 36         this.name = name; 37  } 38 
39     public Teacher getTeacher() { 40         return teacher; 41  } 42 
43     public void setTeacher(Teacher teacher) { 44         this.teacher = teacher; 45  } 46 
47     public List<Student> getStudents() { 48         return students; 49  } 50 
51     public void setStudents(List<Student> students) { 52         this.students = students; 53  } 54 
55  @Override 56     public String toString() { 57         return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher 58                 + ", students=" + students + "]"; 59  } 60 }

 1 <!-- 
 2  根据classId查询对应的班级信息,包括学生,老师  3      -->
 4     <!-- 
 5  方式一: 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集  6  SELECT * FROM class c, teacher t,student s WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND c.c_id=1  7      -->
 8     <select id="getClass3" parameterType="int" resultMap="ClassResultMap3">
 9  select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and c.c_id=#{id} 10     </select>
11     <resultMap type="me.gacl.domain.Classes" id="ClassResultMap3">
12         <id property="id" column="c_id"/>
13         <result property="name" column="c_name"/>
14         <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher">
15             <id property="id" column="t_id"/>
16             <result property="name" column="t_name"/>
17         </association>
18         <!-- ofType指定students集合中的对象类型 -->
19         <collection property="students" ofType="me.gacl.domain.Student">
20             <id property="id" column="s_id"/>
21             <result property="name" column="s_name"/>
22         </collection>
23     </resultMap>
24     
25     <!-- 
26  方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型 27  SELECT * FROM class WHERE c_id=1; 28  SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id的值 29  SELECT * FROM student WHERE class_id=1 //1是第一个查询得到的c_id字段的值 30      -->
31      <select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
32  select * from class where c_id=#{id} 33      </select>
34      <resultMap type="me.gacl.domain.Classes" id="ClassResultMap4">
35         <id property="id" column="c_id"/>
36         <result property="name" column="c_name"/>
37         <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher" select="getTeacher2"></association>
38         <collection property="students" ofType="me.gacl.domain.Student" column="c_id" select="getStudent"></collection>
39      </resultMap>
40      
41      <select id="getTeacher2" parameterType="int" resultType="me.gacl.domain.Teacher">
42  SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id} 43      </select>
44      
45      <select id="getStudent" parameterType="int" resultType="me.gacl.domain.Student">
46  SELECT s_id id, s_name name FROM student WHERE class_id=#{id} 47      </select>

 1 package me.gacl.test;  2 
 3 import me.gacl.domain.Classes;  4 import me.gacl.util.MyBatisUtil;  5 import org.apache.ibatis.session.SqlSession;  6 import org.junit.Test;  7 
 8 public class Test4 {  9     
10  @Test 11     public void testGetClass3(){ 12         SqlSession sqlSession = MyBatisUtil.getSqlSession(); 13         /**
14  * 映射sql的标识字符串， 15  * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值， 16  * getClass3是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL 17          */
18         String statement = "me.gacl.mapping.classMapper.getClass3";//映射sql的标识字符串 19         //执行查询操作，将查询结果自动封装成Classes对象返回
20         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录 21         //使用SqlSession执行完SQL之后需要关闭SqlSession
22  sqlSession.close(); 23         //打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]
24  System.out.println(clazz); 25  } 26     
27  @Test 28     public void testGetClass4(){ 29         SqlSession sqlSession = MyBatisUtil.getSqlSession(); 30         /**
31  * 映射sql的标识字符串， 32  * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值， 33  * getClass4是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL 34          */
35         String statement = "me.gacl.mapping.classMapper.getClass4";//映射sql的标识字符串 36         //执行查询操作，将查询结果自动封装成Classes对象返回
37         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录 38         //使用SqlSession执行完SQL之后需要关闭SqlSession
39  sqlSession.close(); 40         //打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]
41  System.out.println(clazz); 42  } 43 }