第三天Mybatis学习总结——多表开发（一对多,多对多）

一对多开发
这里javaBean就不写代码了，一个是Grades类，一个是学生类
StudentMapper.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">

<!-- namespace属性是名称空间  必须唯一s\ -->
<mapper namespace="studentNamespace">       
    <!-- 当实体属性和表字段名不相同的时候， 必须书写以下代码
    -->
    <resultMap type="onebymany.Student" id="studentMap">
        <id  property="id" column="sid"/>
        <result property="name" column="sname" />
    </resultMap>

    <select id="findAllByName" parameterType="string" resultMap="studentMap">
        select s.sid,s.sname
        from students2 s inner join grades g
        where s.sgid = g.gid 
        and  g.gname = #{name}
    </select>
</mapper>

GradesMapper,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">

<!-- namespace属性是名称空间  必须唯一s\ -->
<mapper namespace="gradesNamespace">        
    <!-- 当实体属性和表字段名不相同的时候， 必须书写以下代码
    -->
    <resultMap type="onebymany.Grades" id="gradesMap">
        <id  property="id" column="gid"/>
        <result property="name" column="gname" />
    </resultMap>
    <select id="findByName" parameterType="string" resultMap="gradesMap">
        select g.gname
        from students2 s inner join grades g
        on s.sgid = g.gid
        and s.sname = #{name}
    </select>
</mapper>

测试类：

package onebymany;

import java.util.List;

import org.apache.ibatis.session.SqlSession;

import util.MybatisUtil;

/**
 * 持久层
 * @author user
 *
 */
public class GradesStudentDao {
    /**
     * 查询java班级学科有哪些学生
     *@param name 学科的名 
     */
    public List<Student> findAllByName(String name) {
        SqlSession sqlSession = null;
        try {
            sqlSession = MybatisUtil.getSqlSession();
            return sqlSession.selectList("studentNamespace.findAllByName", name);
        }catch (Exception e) {
            // TODO: handle exception
            e.printStackTrace();
        }finally {
            MybatisUtil.closeSqlSession();
        }
        return null;
    }

    /***
     * 查询某个学生是哪个学科的学生
     * @param name 代表学生姓名
     * @return
     */
    public Grades findByName(String name) {
        SqlSession sqlSession = null;
        try {
            sqlSession = MybatisUtil.getSqlSession();
            return sqlSession.selectOne("gradesNamespace.findByName", name);
        }catch (Exception e) {
            // TODO: handle exception
            e.printStackTrace();
        }finally {
            MybatisUtil.closeSqlSession();
        }
        return null;
    }   
    public static void main(String[] args) {
        GradesStudentDao dao = new GradesStudentDao();
//      List<Student> list = dao.findAllByName("java");
//      System.out.println("java学科有"+list.size()+"个学生  信息如下：");
//      for (Student student : list) {
//          System.out.println(student.getId()+"   "+student.getName());
//      }
        Grades grades = dao.findByName("张三");
        System.out.println("张三是："+grades.getName()+"学科的");
    }
}

多对多开发

Bean实例和数据库按照上图建立即可
重点看一下配置文件的编写
StudentMapper.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">

<!-- namespace属性是名称空间  必须唯一s\ -->
<mapper namespace="studentNamespace">       
    <!-- 当实体属性和表字段名不相同的时候， 必须书写以下代码
    -->
    <resultMap type="manybymany.Student" id="studentMap">
        <id  property="id" column="sid"/>
        <result property="name" column="sname" />
    </resultMap>
    <select id="findAllByCourseName" parameterType="string" resultMap="studentMap">
        select s.sname 
        from student3 s inner join middles m
        on s.sid = m.msid
        inner join coures c
        on m.mcid = c.cid
        and c.cname = #{name}
    </select>
</mapper>



CourseMapper.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">

<!-- namespace属性是名称空间  必须唯一s\ -->
<mapper namespace="courseNamespace">        
    <!-- 当实体属性和表字段名不相同的时候， 必须书写以下代码
    -->
    <resultMap type="manybymany.Course" id="courseMap">
        <id  property="id" column="cid"/>
        <result property="name" column="cname" />
    </resultMap>
    <select id="findAllByName" parameterType="string" resultMap="courseMap">
        select c.cid,c.cname
        from student3 s inner join middles m
        on s.sid = m.msid
        inner join coures c
        on m.mcid = c.cid
        and s.sname = #{name}
    </select>
</mapper>

测试类：

package manybymany;

import java.util.List;

import org.apache.ibatis.session.SqlSession;

import util.MybatisUtil;

/**
 * 持久层
 * @author user
 *
 */
public class StudentCourseDao {
    /**
     * 查询学生选学啦哪些课程
     *@param name 学生的名字
     */
    public List<Course> findAllByName(String name) {
        SqlSession sqlSession = null;
        try {
            sqlSession = MybatisUtil.getSqlSession();
            return sqlSession.selectList("courseNamespace.findAllByName", name);
        }catch (Exception e) {
            // TODO: handle exception
            e.printStackTrace();
        }finally {
            MybatisUtil.closeSqlSession();
        }
        return null;
    }



    /**
     * 查询课程由哪些学生选修
     *@param name 课程名称
     */
    public List<Student> findAllByCourseName(String name) {
        SqlSession sqlSession = null;
        try {
            sqlSession = MybatisUtil.getSqlSession();
            return sqlSession.selectList("studentNamespace.findAllByCourseName", name);
        }catch (Exception e) {
            // TODO: handle exception
            e.printStackTrace();
        }finally {
            MybatisUtil.closeSqlSession();
        }
        return null;
    }



    public static void main(String[] args) {
        StudentCourseDao dao = new StudentCourseDao();
//      List<Course> courseList = dao.findAllByName("yss");
//      System.out.print("yss选修了"+courseList.size()+"门课程");
//      for (Course course : courseList) {
//          System.out.print("课程信息 = "+course.getName()+"     ");
//      }
        List<Student> studengList = dao.findAllByCourseName("java");
        System.out.print("选修了java课程的人有"+studengList.size()+"个人");
        for (Student student : studengList) {
            System.out.println("姓名"+student.getName());
        }
    }
}

以上就是mybayis一对一、一对多、多对多的sql语句的写法，项目实战中会经常遇到