JDBC及常见数据存取问题

Posted 2020-10-12

 连接数据库步骤：

注册驱动：Class.forName("com.mysql.jdbc.Driver");

设置数据库地址、用户名、密码：String url = "jdbc:mysql://localhost:3306/database";

创建连接：Connection conn = DriverManager.getConnection(url, username, password);

创建sql语句字符串：String sql = "insert into tb_books(name,price,bookCount,author) values(?,?,?,?)";

使用PreparedStatement处理sql语句：PreparedStatement ps = conn.prepareStatement(sql);

设置缺省数据：ps.setString(1, variable);

执行sql语句，返回受影响行数：int row = ps.executeUpdate();

释放对象：ps.close();

关闭连接：conn.close();

    out.println(request.getParameter("price"));
    double price = Double.parseDouble(request.getParameter("price"));
    out.println(price);
    try {
        Class.forName("com.mysql.jdbc.Driver");
        String url = "jdbc:mysql://localhost:3306/test";
        String username = "root";
        String password = "123456";
        Connection conn = DriverManager.getConnection(url, username, password);
        String sql = "insert into tb_books(name,price,bookCount,author) values(?,?,?,?)";
        PreparedStatement ps = conn.prepareStatement(sql);
        ps.setString(1, book.getName());
        ps.setDouble(2, price);
        ps.setInt(3, book.getBookCount());
        ps.setString(4, book.getAuthor());
        int row = ps.executeUpdate();
        if (row > 0) {
            out.print("成功添加了" + row + "条数据！");
        }
        ps.close();
        conn.close();
    } catch (Exception e) {
        out.print("添加失败！");
        e.printStackTrace();
    }

 

    try {
        Class.forName("com.mysql.jdbc.Driver");
        String url = "jdbc:mysql://localhost:3306/test";
        String username = "root";
        String password = "123456";
        Connection conn = DriverManager.getConnection(url, username, password);
        if (conn != null) {
            out.println("success!!");
            conn.close();
        } else {
            out.println("fail");
        }
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (SQLException e) {
        e.printStackTrace();
    }

 

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Q：输入的是小数，但是存入之后显示的是四舍五入后的整数。

A：float最多表示7位有效数据。double最多表示16位有效数据。改了double类型的位数为0，就好了。

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