Mybatis-05-一對多和多對一問題
阿新 • • 發佈:2021-02-16
技術標籤:mybatis
一、多對一:多個學生對應一個老師。學生屬性中關聯老師(物件)
資料庫:
- 學生表:(tid是老師id)
- 老師表:
實體類:
package com.domain;
import lombok.Data;
@Data
public class Student {
private int id;
private String name;
//學生關聯一個老師
private Teacher teacher;
}
package com.domain;
import lombok.Data;
@Data
public class Teacher {
private int id;
private String name;
}
Mapper介面:
- StudentMapper:
package com.dao;
import com.domain.Student;
import java.util.List;
public interface StudentMapper {
//查詢所有學生和對應的老師
List<Student> getStudent();
}
- TeacherMapper:(TeacherMapper這裡我用的是註解,也可以寫相應的xml檔案)
package com.dao;
import com.domain.Teacher;
import org.apache.ibatis.annotations.Select;
public interface TeacherMapper {
@Select("select * from teacher")
Teacher getTeachers();
}
核心配置檔案:
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<!--引入外部配置檔案-->
<properties resource="db.properties"/>
<settings>
<!--標準的日誌工廠實現-->
<setting name="logImpl" value="STDOUT_LOGGING"/>
</settings>
<!--實體類起別名-->
<typeAliases>
<!-- <typeAlias type="com.domain.User" alias="user"/>-->
<package name="com.domain"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
</dataSource>
</environment>
</environments>
<!--指定mapper檔案-->
<mappers>
<mapper class="com.dao.StudentMapper"/>
<mapper class="com.dao.TeacherMapper"/>
</mappers>
</configuration>
如果按原來的查詢方式,即 StudentMapper.xml 檔案中的sql為:
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.dao.StudentMapper">
<select id="getStudent" resultType="student">
select * from student
</select>
</mapper>
這樣得到的結果中,Teacher屬性為null,結果如下:
所以為了得到學生的所有資訊,包括對應的老師,需要對 StudentMapper.xml 檔案中的語句進行相應的處理。處理方式有兩種。
方式一:按照結果巢狀處理,即直接查詢出結果,再進行結果集的對映
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.dao.StudentMapper">
<!--
多對一的處理
要把多個學生及對應的老師給顯示出來
-->
<select id="getStudents2" resultMap="studentTeacher2">
select s.id sid,s.name sname,t.id id, t.name tname from student s join teacher t on s.tid = t.id
</select>
<resultMap id="studentTeacher2" type="Student">
<result property="id" column="sid"/>
<result property="name" column="sname" />
<!--property中是學生的屬性,型別是Teacher型別-->
<association property="teacher" javaType="Teacher">
<!--property中是老師的屬性name,column是資料庫查出來的欄位-->
<result property="id" column="id"/>
<result property="name" column="tname"/>
</association>
</resultMap>
note:association和collection都用來關聯屬性,但是association是對物件進行操作,而collection是對集合進行操作!
方式二:按查詢巢狀處理
1. 先查詢所有的學生資訊
2. 再根據查詢出來的學生的tid,尋找對應的老師(子查詢)
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.dao.StudentMapper">
<select id="getStudents" resultMap="studentTeacher">
select * from student;
</select>
<resultMap id="studentTeacher" type="Student">
<!--student中的屬性-->
<result column="id" property="id"/>
<result column="name" property="name"/>
<!--
物件:association 集合:collection
javaType:把sql語句查詢出的結果集封裝給某個類的物件(可以省略)
select: 下一條要執行的sql語句(或者說當前屬性是呼叫select指定的方法查出的結果)
property:實體類中的屬性
column:資料庫中對應的屬性
-->
<association property="teacher" column="tid" javaType="Teacher" select="getTeachers"/>
</resultMap>
<select id="getTeachers" resultType="teacher">
select * from teacher where id = #{id}
</select>
</mapper>
測試類:
package com.dao;
import com.domain.Student;
import com.domain.Teacher;
import com.utils.MyBatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import java.util.List;
public class Mytest {
@Test
public void test(){
SqlSession sqlSession = MyBatisUtils.getSqlSession();
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
List<Student> studentList = mapper.getStudent();
for (Student student : studentList) {
System.out.println(student);
}
}
}
這兩種方式都能得到想要的結果:
總結:
這兩種方式都用到了Map對映,方式一的sql語句複雜一點,但是隻要能把sql語句寫出來,再做一個map對映就可以。方式二sql語句簡單,但是進行對映時比較複雜,而且還要寫出來查詢teacher的sql語句。
二、一對多:一個老師擁有多個學生,這就成了老師屬性中關聯一群學生(集合)
實體類:
package com.domain;
import lombok.Data;
import java.util.List;
@Data
public class Teacher {
private int id;
private String name;
//一個老師擁有多個學生
private List<Student> students;
}
package com.domain;
import lombok.Data;
@Data
public class Student {
private int id;
private String name;
private int tid;
}
Mapper介面:
- StudentMapper:
package com.dao;
import com.domain.Student;
import org.apache.ibatis.annotations.Select;
import java.util.List;
public interface StudentMapper {
@Select("select * from student")
List<Student> getStudents();
}
- TeacherMapper:
package com.dao;
import com.domain.Teacher;
import org.apache.ibatis.annotations.Param;
public interface TeacherMapper {
//獲取老師
// Teacher getTeacher();
//獲取指定老師下的所有學生及老師資訊
Teacher getTeacher(@Param("tid") int id);
}
核心配置檔案: 和多對一的相同
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<!--引入外部配置檔案-->
<properties resource="db.properties"/>
<settings>
<!--標準的日誌工廠實現-->
<setting name="logImpl" value="STDOUT_LOGGING"/>
</settings>
<!--實體類起別名-->
<typeAliases>
<!-- <typeAlias type="com.domain.User" alias="user"/>-->
<package name="com.domain"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper class="com.dao.StudentMapper"/>
<mapper class="com.dao.TeacherMapper"/>
</mappers>
</configuration>
Mapper.xml檔案:
- StudentMapper.xml:因為用的是註解,所以不用配置它的xml檔案
- 和一對多類似,TeacherMapper.xml的配置也有兩種方式。
方式一:按結果巢狀查詢
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.dao.TeacherMapper">
<!--按結果巢狀查詢-->
<select id="getTeacher" resultMap="TeacherStudent">
select t.id tid ,t.name tname, s.name sname, s.id sid from teacher t join student s on t.id = s.tid where t.id = #{tid}
</select>
<resultMap id="TeacherStudent" type="teacher">
<result property="id" column="tid"/>
<result property="name" column="tname"/>
<!--老師中的學生屬性是集合,用collection,
javaType和ofType都是用來指定物件型別的
JavaType是用來指定實體類pojo中屬性的型別
ofType指定的是對映到list集合屬性中pojo的型別。
-->
<collection property="students" javaType="ArrayList" ofType="Student">
<result property="name" column="sname"/>
<result property="id" column="sid"/>
</collection>
</resultMap>
</mapper>
方式二:按子查詢
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.dao.TeacherMapper">
<!--按按查詢巢狀處理-->
<select id="getTeacher2" resultMap="TeacherStudents">
select * from teacher where id = #{tid};
</select>
<resultMap id="TeacherStudents" type="teacher">
<!--column是一對多的外來鍵 , 寫的是一的主鍵的列名-->
<collection property="students" column="id" javaType="ArrayList" ofType="Student" select="getStudents"/>
</resultMap>
<select id="getStudents" resultType="Student">
select * from student where tid = #{tid}
</select>
</mapper>
測試:
@Test
public void testTeacherMapper(){
SqlSession sqlSession = MyBatisUtils.getSqlSession();
TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
Teacher teacher = mapper.getTeacher(1);
System.out.println(teacher);
}
結果:
Teacher(id=1, name=村長, students=[Student(id=1, name=美羊羊, tid=0),
Student(id=2, name=紅太狼, tid=0), Student(id=3, name=喜羊羊, tid=0),
Student(id=4, name=懶羊羊, tid=0), Student(id=5, name=灰太狼, tid=0)])
總結:
其實不管是一對多還是多對一,主要考察的還是sql語句,和map對映,掌握好這兩個就好。