MyBatis in many

Many understand:

• A teacher with more students
• If a teacher here for, is to many a phenomenon that has a group of students from a teacher below (collection)!

Database can refer to many-to .

Example:

The following example is built in SpringBoot environment. There are two ways to carry out a many queries.

Like nested query processing in accordance with sub-query in SQL
nested SQL query processing as contingency tables in accordance with the results

To cite just one kind, the nesting process in accordance with the result of contingency tables as SQL queries. It is better understood.

1. pom.xml dependence

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.mybatis.spring.boot</groupId>
            <artifactId>mybatis-spring-boot-starter</artifactId>
            <version>2.1.1</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <scope>test</scope>
        </dependency>

2. application.properties

mybatis.mapper-locations=classpath:mapper/*.xml
mybatis.type-aliases-package=com.cc.pojo
mybatis.config-location=classpath:mybatis-config.xml
 
spring.datasource.username=root
spring.datasource.password=123456
spring.datasource.url=jdbc:mysql://localhost:3306/mis?serverTimezone=Asia/Shanghai&useUnicode=true&characterEncoding=utf-8

3. The configuration log (log standard used here mybatis .STDOUT_LOGGING.)

mybatis-config.xml

<?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>
    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING" />
    </settings>
</configuration>

4 major code

1) Student under the category pojo package, Teacher class.

package com.cc.pojo; 

package com.cc.pojo;

import lombok.Data;

public class Student {
    private int id;
    private String name;
    private int tid;
    //GET,SET,ToString，有参，无参构造
}

package com.cc.pojo;

import java.util.List;

public class Teacher {
    private int id;
    private String name;
    //一个老师多个学生
    private List<Student> students;
}

2) at the interface mapper.

package com.cc.mapper;
 

package com.cc.mapper;

import org.apache.ibatis.annotations.Mapper;

@Mapper
public interface StudentMapper {

}

package com.cc.mapper;

import com.cc.pojo.Teacher;
import org.apache.ibatis.annotations.Mapper;

@Mapper
public interface TeacherMapper {
    //获取指定老师，及老师下的所有学生
    public Teacher getTeacher(int id);
}

3) an interface corresponding to the write Mapper profile mapper.xml 

<?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.cc.mapper.StudentMapper">

</mapper>

<?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.cc.mapper.TeacherMapper">
        <!--
        思路:
            1. 从学生表和老师表中查出学生id，学生姓名，老师姓名
            2. 对查询出来的操作做结果集映射
                1. 集合的话，使用collection！
                    JavaType和ofType都是用来指定对象类型的
                    JavaType是用来指定pojo中属性的类型
                    ofType指定的是映射到list集合属性中pojo的类型。
        -->
        <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid, s.name sname , t.name tname, t.id tid
        from student s,teacher t
        where s.tid = t.id and t.id=#{tid}
    </select>

        <resultMap id="TeacherStudent" type="Teacher">
            <result property="id" column="tid"/>
            <result  property="name" column="tname"/>
            <collection property="students" ofType="Student">
                <result property="id" column="sid" />
                <result property="name" column="sname" />
                <result property="tid" column="tid" />
            </collection>
        </resultMap>
</mapper>

4) Test

package com.cc;

import com.cc.mapper.TeacherMapper;
import com.cc.pojo.Teacher;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

@RunWith(SpringRunner.class)
@SpringBootTest
public class TeacherTest {

    @Autowired
    private TeacherMapper teacherMapper;

    @Test
    public void testTeacher() {
        Teacher teacher = teacherMapper.getTeacher(2);
        System.out.println(teacher);
    }
}

5) operating results

Teacher{id=2, name='大老师', students=[Student(id=1, name=小明, tid=2), Student(id=2, name=小红, tid=2), Student(id=3, name=小张, tid=2), Student(id=4, name=小李, tid=2), Student(id=5, name=小王, tid=2)]}

 

Note: resultMap mapped attributes can not be less time, or else lose out.

 

For many-to-many and summary:

1. Associated -association
2. -Collection collection
3. So the association is many-to-one and, while the collection is for one to many relationship

4. JavaType ofType and are used to specify the type of the object

   • JavaType attribute is used to specify the type of pojo
   • ofType specified list is mapped to a set of attribute types of pojo.