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mybatis一对多嵌套查询
一个class中有许多student，属于一对多关系。
本文利用mybatis注解，嵌套查询方式来实现一对多查询，在根据classId查询class时，顺便查询并展示出class中的student
student实体类
package com.itheima.dao;public class StudentDao { private Integer id; private String name; private Integer age; private Integer cid; public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Integer getAge() { return age; } public void setAge(Integer age) { this.age = age; } public Integer getCid() { return cid; } public void setCid(Integer cid) { this.cid = cid; } @Override public String toString() { return "StudentDao{" + "id=" + id + ", name='" + name + '\'' + ", age=" + age + ", cid=" + cid + '}'; }}class实体类
package com.itheima.dao;import java.util.List;public class ClassDao { private Integer id; private String className; private List<StudentDao> studentsList; public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getClassName() { return className; } public void setClassName(String className) { this.className = className; } @Override public String toString() { return "ClassDao{" + "id=" + id + ", className='" + className + '\'' + ", studentsList=" + studentsList + '}'; }}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> <!--设置连接数据库的环境--> <environments default="development"> <environment id="development"> <transactionManager type="JDBC"/> <dataSource type="POOLED"> <property name="driver" value="com.mysql.jdbc.Driver"/> <property name="url" value="jdbc:mysql://localhost:3306/mybatis"/> <property name="username" value="root"/> <property name="password" value="root"/> </dataSource> </environment> </environments> <!--引入映射文件--> <mappers> <mapper class="com.itheima.mapper.StudentMapper"/> <mapper class="com.itheima.mapper.ClassMapper"/> </mappers></configuration>studentMapper.java
package com.itheima.mapper;import com.itheima.dao.StudentDao;import org.apache.ibatis.annotations.Many;import org.apache.ibatis.annotations.Result;import org.apache.ibatis.annotations.Results;import org.apache.ibatis.annotations.Select;import java.util.List;public interface StudentMapper { @Select("select * from student where cid = #{cid}") @Results({@Result(id = true, property = "id", column = "id"), @Result(property = "name",column = "name"), @Result(property = "age",column = "age"), @Result(property = "cid", column = "cid")}) List<StudentDao> selectStudentByCid(Integer cid);}classMapper.java
package com.itheima.mapper;import com.itheima.dao.ClassDao;import org.apache.ibatis.annotations.Many;import org.apache.ibatis.annotations.Result;import org.apache.ibatis.annotations.Results;import org.apache.ibatis.annotations.Select;import java.util.List;public interface ClassMapper { @Select("select * from class where id = #{id}") @Results({@Result(id =true, property = "id",column = "id"), @Result(property = "className",column = "className"), @Result(property ="studentsList",column = "id", many = @Many(select = "com.itheima.mapper.StudentMapper.selectStudentByCid"))}) ClassDao selectClassById(Integer id);}testMybatis.java
import com.itheima.dao.ClassDao;import com.itheima.dao.StudentDao;import com.itheima.mapper.ClassMapper;import com.itheima.mapper.StudentMapper;import org.apache.ibatis.annotations.Mapper;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import org.junit.Test;import java.io.IOException;import java.io.InputStream;import java.util.List;public class testMybatis { @Test public void test() throws IOException { InputStream is = Resources.getResourceAsStream("mybatis-config.xml"); SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder(); SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is); SqlSession sqlSession = sqlSessionFactory.openSession(true); ClassMapper classMapper = sqlSession.getMapper(ClassMapper.class); ClassDao res = classMapper.selectClassById(2); System.out.println(res.toString()); }}数据库sql文件
INSERT INTO `class` (`id`, `className`) VALUES (1, '软件19-1');INSERT INTO `class` (`id`, `className`) VALUES (2, '软件19-2');INSERT INTO `student` (`name`, `age`, `cid`, `id`) VALUES ('罗辑', 18, 2, 1);INSERT INTO `student` (`name`, `age`, `cid`, `id`) VALUES ('张三', 18, 2, 2);INSERT INTO `student` (`name`, `age`, `cid`, `id`) VALUES ('李四', 18, 2, 3);查询结果可看出，2班的班级信息和2班的学生信息
注意事项：
一对多关系中，多方实体类存一的那方的主键作为外键，‘’一‘’的那方的实体类存‘’多‘’的那方的 list<多的那方的实体类属性>以便查询时展示出来。
在“一”的那方的mapper接口中定义嵌套查询。
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