实现联表查询

Posted 2022-09-15 kfsrex

一、一对一关联

1.在数据库中创建班级表和老师表

CREATE TABLE teacher( t_id INT PRIMARY KEY AUTO_INCREMENT, t_name VARCHAR(20) ); CREATE TABLE class( c_id INT PRIMARY KEY AUTO_INCREMENT, c_name VARCHAR(20), teacher_id INT ); ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id); INSERT INTO teacher(t_name) VALUES(‘LS1‘); INSERT INTO teacher(t_name) VALUES(‘LS2‘); INSERT INTO class(c_name, teacher_id) VALUES(‘bj_a‘, 1); INSERT INTO class(c_name, teacher_id) VALUES(‘bj_b‘, 2);

 

2.创建相应的实体类Classes和Teacher

 

3.配置映射文件ClassesMapper.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.zhiyou100.kfs.dao.ClassesMapper">        <!-- 通过cid查询班级信息和对应的老师信息 -->               <resultMap type="com.zhiyou100.kfs.bean.Classes" id="BaseMap">               <!--                      id标签数据库表主键对应实体类属性                      result标签数据库表除了主键对应的实体类属性                             column属性是对应数据库表的栏，property属性是对应实体类的属性               -->               <id column="c_id" property="cid"/>               <result column="c_name" property="cname"/>               <result column="teacher_id" property="teacherid"/>               <!-- association标签是一对一关联返回的配置标签                      property属性是实体类中的对象名，javaType是对象的全类名                -->               <association property="teacher" javaType="com.zhiyou100.kfs.bean.Teacher">                      <id column="t_id" property="tid"/>                      <result column="t_name" property="tname"/>               </association>        </resultMap>        <!-- 1.直接用联表sql语句查询，返回结果集再配置一下 -->        <select id="select1" resultMap="BaseMap">               select * from class c,teacher t where c.teacher_id=t.t_id and c.c_id=#cid        </select> </mapper>

 

 

4.创建dao接口ClassesMapper.java

public interface ClassesMapper        /**         * 一对一关联         * @param cid         * @return         */        public Classes select1(int cid);

 

5.创建单元测试

@Test          void testSelect1() throws IOException                    //1.获取配置文件config.xml加入到io流中                    InputStream inputStream = Resources.getResourceAsStream("config.xml");                    //2.通过SqlSessionFatoryBuilder的builder创建sqlsessionfatory                    SqlSessionFactory sqlSessionFatory = new SqlSessionFactoryBuilder().build(inputStream);                    //3.通过sqlSessionFactory的openSession方法创建SqlSession                    SqlSession sqlSession=sqlSessionFatory.openSession();                    //4.通过sqlSession的getMapper传入dao全类名的方法创建UserMapper对象                    ClassesMapper um=sqlSession.getMapper(com.zhiyou100.kfs.dao.ClassesMapper.class);                    //5.调用UserMapper的selectById来使用通过id查询一个用户                    Classes classes=um.select1(1);                    System.out.println(classes);                    //最后一个页是很重要的一个：关闭资源                    sqlSession.close();

 

 

二、一对多关联

1.在数据库里创建学生表

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(‘xs_A‘, 1); INSERT INTO student(s_name, class_id) VALUES(‘xs_B‘, 1); INSERT INTO student(s_name, class_id) VALUES(‘xs_C‘, 1); INSERT INTO student(s_name, class_id) VALUES(‘xs_D‘, 2); INSERT INTO student(s_name, class_id) VALUES(‘xs_E‘, 2); INSERT INTO student(s_name, class_id) VALUES(‘xs_F‘, 2);

 

2.创建对应Student实体类

3.配置映射文件ClassesMapper.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.zhiyou100.kfs.dao.ClassesMapper">        <!-- 通过cid查询班级信息和对应的学生信息 -->               <resultMap type="com.zhiyou100.kfs.bean.Classes" id="BaseMap">               <!--                      id标签数据库表主键对应实体类属性                      result标签数据库表除了主键对应的实体类属性                             column属性是对应数据库表的栏，property属性是对应实体类的属性               -->               <id column="c_id" property="cid"/>               <result column="c_name" property="cname"/>               <result column="teacher_id" property="teacherid"/>               <!-- collection标签是一对多关联的配置                      property是实体类对应的集合名，ofType是集合类的全类名                -->               <collection property="list" ofType="com.zhiyou100.kfs.bean.Student">                      <id column="s_id" property="sid"/>                      <result column="s_name" property="sname"/>                      <result column="class_id" property="classid"/>               </collection>        </resultMap>        <!-- 1.直接用联表sql语句查询，返回结果集再配置一下 -->        <select id="select2" resultMap="BaseMap">               select * from class c,student s where c.c_id=s.class_id and c.c_id=#cid        </select> </mapper>

 

4.创建dao接口ClassesMapper.java

public interface ClassesMapper        /**         * 一对多关联         * @param cid         * @return         */        public Classes select2(int cid);

5.创建单元测试来测试

@Test          void testSelect2() throws IOException                    //1.获取配置文件config.xml加入到io流中                    InputStream inputStream = Resources.getResourceAsStream("config.xml");                    //2.通过SqlSessionFatoryBuilder的builder创建sqlsessionfatory                    SqlSessionFactory sqlSessionFatory = new SqlSessionFactoryBuilder().build(inputStream);                    //3.通过sqlSessionFactory的openSession方法创建SqlSession                    SqlSession sqlSession=sqlSessionFatory.openSession();                    //4.通过sqlSession的getMapper传入dao全类名的方法创建UserMapper对象                    ClassesMapper um=sqlSession.getMapper(com.zhiyou100.kfs.dao.ClassesMapper.class);                    //5.调用UserMapper的selectById来使用通过id查询一个用户                    Classes classes=um.select2(1);                    System.out.println(classes);                    //最后一个页是很重要的一个：关闭资源                    sqlSession.close();