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  • MyBatis_2

    使用接口结合xml文件

    数据库表与实体类创建

    
    create database mybatis;
    use mybatis;
    CREATE TABLE users(id INT PRIMARY KEY AUTO_INCREMENT, NAME
    VARCHAR(20), age INT);
    INSERT INTO users(NAME, age) VALUES('Tom', 12);
    INSERT INTO users(NAME, age) VALUES('Jack', 11);
    
    
    
    public class User {
        private int id;
        private String name;
        private int age;
        //get,set 方法 无参与有参的构造方法创建
    }
    
    

    Users的操作类

    public interface UsersDao{
        public Users getUser(int id)
    }
    

    Users映射文件

    新建映射文件UsersMapper

    <?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">
    //com.huangwei.lb包名
    <mapper namespace="com.huangwei.lb.dao.UsersDao">
        <select id="getUser" paramterType="int" resultType="com.huangwei.lb.bean.Users">
            select * from users where id = #{id}
        </select>
    </mapper>
    

    测试

    新建一个测试类

    //com.huangwei.lb包名
    class UsersTest{
        static SqlSession session = null;
        final String str = "com.huangwei.lb.mapper.UsersMapper.";
        static UsersDao usesDao = null;
        @BeforeAll
        static void setUpBeforeClass() throws Exception {
              Reader reader = Resources.getResourceAsReader("config.xml");
              SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(reader);
              session = sessionFactory.openSession();
              usersDao = session.getMapper(UsersDao.class);
         }
        @Test
        void testFindAll(){
            Users users = usersDao.getUser(1);
        }
    }
    

    解决字段与属性不匹配(以Order表为例)

    数据库表与实体类创建

    
    CREATE TABLE orders(
    order_id INT PRIMARY KEY AUTO_INCREMENT,
    order_no VARCHAR(20),
    order_price FLOAT
    );
    INSERT INTO orders(order_no, order_price) VALUES('aaaa', 23);
    INSERT INTO orders(order_no, order_price) VALUES('bbbb', 33);
    INSERT INTO orders(order_no, order_price) VALUES('cccc', 22);
    
    
    
    public class Order {
        private int id;
        private String orderNo;
        private float price;
       //get,set 方法 无参与有参的构造方法创建
    }
    
    

    通过给查询字段起别名,要求别名与属性名一致

    
    <select id="selectOrder" parameterType="int" resultType="_Order">
        select order_id id, order_no orderNo,order_price price from orders where order_id=#{id}
    </select>
    
    

    使用resultMap标签来定义实体类与字段之间的关系

    
    <select id="selectOrderResultMap" parameterType="int" resultMap="orderResultMap">
        select * from orders where order_id=#{id}
    </select>
    <resultMap type="_Order" id="orderResultMap">
        <id property="id" column="order_id"/>
        <result property="orderNo" column="order_no"/>
        <result property="price" column="order_price"/>
    </resultMap>
    
    

    实现链表查询【根据班级ID查询班级信息(带有老师信息)】

    数据库表与实体类创建

    
    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);
    
    
    
    public class Teacher {
        private int id;
        private String name;
    //get,set 方法 无参与有参的构造方法创建
    }
    public class Classes {
        private int id;
        private String name;
        private Teacher teacher;
    //get,set 方法 无参与有参的构造方法创建
    }
    
    

    定义 sql 映射文件 ClassMapper.xml

    方法一:通过联表查询

    
    <select id="getClass" parameterType="int" resultMap="ClassResultMap">
        select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
    </select>
    <resultMap type="_Classes" id="ClassResultMap">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" javaType="_Teacher">
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
    </resultMap>
    
    

    方法二:通过嵌套查询

    
    <select id="getClass2" parameterType="int" resultMap="ClassResultMap2">
        select * from class where c_id=#{id}
    </select>
    <resultMap type="_Classes" id="ClassResultMap2">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" javaType="_Teacher" select="getTeacher"></association>
    </resultMap>
    <select id="getTeacher" parameterType="int" resultType="_Teacher">
        SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
    </select>
    
    

    19c2837969000cb1398c3947d7b74313.png

    测试

    @Test
    public void testOO() {
    SqlSession sqlSession = factory.openSession();
    Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass",1);
    System.out.println(c);
    }
    @Test
    public void testOO2() {
    SqlSession sqlSession = factory.openSession();
    Classes c =
    sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass2", 1);
    System.out.println(c);
    }
    
    

    实现一对多关联【根据 classId 查询对应的班级信息,包括学生,老师】

    数据库表与实体类创建

    
    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);
    
    
    
    public class Student {
        private int id;
        private String name;
        //get,set 方法 无参与有参的构造方法创建
    }
    public class Classes {
        private int id;
        private String name;
        private Teacher teacher;
        private List<Student> students;
        //get,set 方法 无参与有参的构造方法创建
    }
    
    

    定义 sql 映射文件 ClassMapper.xml

    **方法一:联表查询

    
    <select id="getClass3" parameterType="int" resultMap="ClassResultMap3">
        select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and c.c_id=#{id}
    </select>
    <resultMap type="_Classes" id="ClassResultMap3">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" javaType="_Teacher">
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
    <!-- ofType 指定 students 集合中的对象类型 -->
        <collection property="students" ofType="_Student">
            <id property="id" column="s_id"/>
            <result property="name" column="s_name"/>
        </collection>
    </resultMap>
    
    

    **方法二:嵌套查询

    
    <select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
        select * from class where c_id=#{id}
    </select>
    <resultMap type="_Classes" id="ClassResultMap4">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" javaType="_Teacher" select="getTeacher2"></association>
        <collection property="students" ofType="_Student" column="c_id" select="getStudent"></collection>
    </resultMap>
    <select id="getTeacher2" parameterType="int" resultType="_Teacher">
        SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
    </select>
    <select id="getStudent" parameterType="int" resultType="_Student">
        SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
    </select>
    
    

    测试

    
    @Test
    public void testOM() {
        SqlSession sqlSession = factory.openSession();
        Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass3",1);
        System.out.println(c);
    }
    @Test
    public void testOM2() {
        SqlSession sqlSession = factory.openSession();
        Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass4",1);
        System.out.println(c);
    }
    
    

    462ed527a44571647a5159280a4936e8.png

    $与#的区别

    "$"中的变量就是直接替换成值的,实际用于字符串拼接。
    一般用于传入数据库对象
    "#" 则是根据变量的类型来替换
    一般用于传入增删改查的where条件

    在同时可用#和$是,建议使用#

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  • 原文地址:https://www.cnblogs.com/DT-Demo/p/11432020.html
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