"jdbc:mysql://127.0.0.1:3306/school_db?useSSL=false&serverTimezone=UTC&characterEncoding=UTF-8"

	// 1. 注册驱动 (JDBC 4.0 以后这一步可以省略)
	Class.forName("com.mysql.cj.jdbc.Driver"); 
	
	// 2. 通过 DriverManager 获取连接
	String url = "jdbc:mysql://localhost:3306/test_db";
	Connection conn = DriverManager.getConnection(url, "root", "password");

	Connection conn = null;
	try {
	    conn = DriverManager.getConnection(url, user, pass);
	    // 1. 关闭自动提交，开启事务
	    conn.setAutoCommit(false);
	
	    // 2. 执行 SQL A (扣钱)
	    // 3. 执行 SQL B (加钱)
	
	    // 4. 业务全部成功，手动提交
	    conn.commit();
	} catch (Exception e) {
	    // 5. 一旦出异常，全部回滚
	    if (conn != null) {
	        try {
	            conn.rollback();
	        } catch (SQLException ex) {
	            ex.printStackTrace();
	        }
	    }
	} finally {
	    // 6. 记得关闭连接
	    if (conn != null) conn.close();
	}

	// 1. 通过连接创建一个邮差
	Statement stmt = conn.createStatement();
	
	// 2. 让邮差去送信（执行 SQL）
	String sql = "UPDATE user SET name = '张三' WHERE id = 1";
	int rows = stmt.executeUpdate(sql); // 返回受影响的行数
	
	// 3. 如果是查询，会带回一个结果集
	ResultSet rs = stmt.executeQuery("SELECT * FROM user");

    while (rs.next()) {
        // 处理当前行的数据
        rs.getXxx(参数);
    }

	Connection conn = DriverManager.getConnection(url,username,password);  
	
	String sql = "SELECT * FROM users";  
	try (conn; Statement stmt = conn.createStatement(); ResultSet rs = stmt.executeQuery(sql)) {  
	    while (rs.next()){  
	        String user = rs.getString("username");  
	        String passwd = rs.getString("password");  
	        String date = rs.getString("created_at");  
	        System.out.println(user + " " + passwd + " " + date);  
	    }  
	}

	String sql = "SELECT * FROM users WHERE username = ? AND password = ?";
	try (PreparedStatement pstmt = conn.prepareStatement(sql)) {
	    // 1. 设置参数（索引从 1 开始）
	    pstmt.setString(1, "admin");
	    pstmt.setString(2, "123456");
	    
	    // 2. 执行查询
	    try (ResultSet rs = pstmt.executeQuery()) {
	        while (rs.next()) {
	            // 处理结果
	        }
	    }
	}

	String sql = "INSERT INTO students (name, score) VALUES (?, ?)";
	try (PreparedStatement pstmt = conn.prepareStatement(sql)) {
	    for (Student s : studentList) {
	        pstmt.setString(1, s.getName());
	        pstmt.setInt(2, s.getScore());
	        pstmt.addBatch(); // 添加到批处理
	    }
	    pstmt.executeBatch(); // 一次性发送所有数据
	}

	// 1. 加载配置文件
	Properties props = new Properties();
	props.load(new FileInputStream("druid.properties"));
	
	// 2. 获取数据源 (DataSource)
	DataSource ds = DruidDataSourceFactory.createDataSource(props);
	
	// 3. 从池中获取连接
	try (Connection conn = ds.getConnection()) {
	    // 这里的 close() 只是把连接归还给池
	    System.out.println("获取到连接：" + conn);
	}

-- 输入参数 name = "Zhang"
SELECT * FROM user WHERE username = #{name}
-- 解析后的 SQL：SELECT * FROM user WHERE username = ?
-- 执行时：SELECT * FROM user WHERE username = 'Zhang'

Mapper接口：

	@Insert("INSERT INTO user(username,password,name,age) VALUES(#{username},#{password},#{name},#{age})")  
	public void insert(User user);//插入用户信息
	
Spring测试：

	@Test  
	public void testInsert(){  
    User user = new User(null,"jjj","666888","鸡鸡鸡",18);  
    userMapper.insert(user);//调用UserMapper接口中的insert方法，插入用户信息  
	}

Mapper接口：

	@Delete("DELETE FROM user WHERE id = #{id}")  
	public void deleteById(Integer id);
	
Spring测试：

	@Autowired  
	private UserMapper userMapper;//注入UserMapper接口的代理对象
	@Test  
	public void testDeleteById(){  
	    userMapper.deleteById(5);  
	}

Mapper接口：

	@Update("UPDATE user set username = #{username},password = #{password},name = #{name},age = #{age} WHERE id = #{id}")  
	public void update(User user);
	
Spring测试：

	public void testUpdate(){  
    User user = new User(1,"zhouyu","8989","周瑜",19);  
    userMapper.update(user);//调用UserMapper接口中的update方法，更新用户信息  
	}

	@Select("SELECT * FROM user WHERE username = #{username} AND password = #{password}")  
	public User findByUsernameAndPassword(  
        @Param("username")String username,  
        @Param("password") String password);

<?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.example.mapper.UserMapper">

    <select id="findById" resultType="com.example.pojo.User">
        SELECT * FROM user WHERE id = #{id}
    </select>

</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="org.example.mapper.UserMapper">  
  
    <select id="findAll" resultType="org.example.pojo.User"> #往User封装  
        SELECT id,username,password,name,age FROM user  
    </select>  
</mapper>
相当于Mapper中的：@Select("SELECT id,username,password,name,age FROM user")

	# 指定XML映射文件的位置  
	mybatis.mapper-locations=classpath:mapper/*.xml

<mapper namespace="asia.creat.mapper.EmpMapper">  
	<select id="listByCondition" resultType="asia.creat.pojo.Emp">  
        SELECT *FROM emp e LEFT JOIN dept d ON e.dept_id = d.id WHERE e.name  
            LIKE CONCAT('%',#{name},'%') AND e.gender = #{gender}            
            AND e.entry_date BETWEEN #{begin} AND #{end}            
            ORDER BY e.update_time DESC 
	</select>  
</mapper>

<select id="find" resultType="Emp">
    SELECT * FROM emp WHERE 1=1
    <if test="name != null">
        AND name LIKE #{name}
    </if>
    <if test="gender != null">
        AND gender = #{gender}
    </if>
</select>

<select id="find" resultType="Emp">
    SELECT * FROM emp
    <where>
        <if test="name != null">AND name = #{name}</if>
        <if test="gender != null">AND gender = #{gender}</if>
    </where>
</select>

<update id="update">
    UPDATE emp
    <set>
        <if test="username != null">username = #{username},</if>
        <if test="password != null">password = #{password},</if>
    </set>
    WHERE id = #{id}
</update>

<delete id="deleteByIds">
    DELETE FROM emp WHERE id IN
    <foreach collection="ids" item="id" open="(" close=")" separator=",">
        #{id}
    </foreach>
</delete>

server:
  port: 8080    # 注意冒号后的空格
  address: 127.0.0.1

student:
  name: ZhangKun
  age: 20
  major: SoftwareEngineering

subjects:
  - Java
  - MyBatis
  - SpringBoot