《PCA 原理推导》18-5线性变换生成的随机变量y_i和y_j的协方差 公式解析

起因， 目的:

如题。 也很简单。

过程:

1. UserTableTest.java

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class UserTableTest {

    private static final String URL = "jdbc:sqlite:test.db"; // 数据库文件名

    public static void main(String[] args) {
        createTable();
        insertData();
        readData();
    }

    private static void createTable() {
        String createTableSQL = "CREATE TABLE IF NOT EXISTS User ("
                + "userId VARCHAR(50) PRIMARY KEY,"
                + "userCode VARCHAR(50),"
                + "userName VARCHAR(50),"
                + "userType INTEGER,"
                + "password VARCHAR(50),"
                + "phone VARCHAR(50),"
                + "email VARCHAR(50),"
                + "remarks VARCHAR(50),"
                + "createdId VARCHAR(50),"
                + "createName VARCHAR(50),"
                + "createTime DATE,"
                + "deleteMark INTEGER"
                + ")";

        try (Connection conn = DriverManager.getConnection(URL);
             Statement stmt = conn.createStatement()) {
            stmt.execute(createTableSQL);
            System.out.println("表创建成功或已存在。");
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    private static void insertData() {
        String sql = "INSERT INTO User (userId, userCode, userName, userType, password, phone, email, remarks, createdId, createName, createTime, deleteMark) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)";
        try (Connection conn = DriverManager.getConnection(URL);
             PreparedStatement pstmt = conn.prepareStatement(sql)) {
            pstmt.setString(1, "001");
            pstmt.setString(2, "user001");
            pstmt.setString(3, "张三");
            pstmt.setInt(4, 1);
            pstmt.setString(5, "password123");
            pstmt.setString(6, "13800138000");
            pstmt.setString(7, "zhangsan@example.com");
            pstmt.setString(8, "这是一条备注");
            pstmt.setString(9, "creator001");
            pstmt.setString(10, "张三");
            pstmt.setDate(11, java.sql.Date.valueOf("2024-11-12"));
            pstmt.setInt(12, 0);

            pstmt.executeUpdate();
            System.out.println("插入成功");
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    private static void readData() {
        String sql = "SELECT * FROM User";
        try (Connection conn = DriverManager.getConnection(URL);
             Statement stmt = conn.createStatement();
             ResultSet rs = stmt.executeQuery(sql)) {

            while (rs.next()) {
                System.out.println("ID: " + rs.getString("userId"));
                System.out.println("用户名: " + rs.getString("userName"));
                // 打印其他字段...
            }
            System.out.println("数据读取成功。");
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

输出:

2. pom.xml

添加数据库驱动之后，一定要更新项目的环境依赖：

右键，选择 “Maven” -> “Reload Project”

 <dependencies>

        <!--  添加一个外部库，然后     -->
        <!--  键点击项目并选择 "Maven" -> "Reload Project"     -->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.13.2</version>
        </dependency>
        
          <!-- 新增到这里！     -->
        <dependency>
            <groupId>org.xerial</groupId>
            <artifactId>sqlite-jdbc</artifactId>
            <version>3.36.0.3</version>
        </dependency>

    </dependencies>

结论 + todo

java. 是国内最流行的语言？

老哥留步，支持一下。