MyBatis学习总结_05_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 }