我在 C# 应用程序中使用 Access 数据库。我需要一些帮助来减去查询中的时间。这是我的代码：
DateTime TimePlus = DateTime.Now.AddMinutes(30);
DateTime now = DateTime.Now;
string Plus30Min = TimePlus.ToString("hh:mm tt");
string timeNow = now.ToString("hh:mm tt");
command.Parameters.Add("tagNo", OleDbType.Integer).Value = tagNo;
command.Parameters.Add("Plus30Min", OleDbType.VarChar).Value = Plus30Min;
command.Parameters.Add("timeNow", OleDbType.VarChar).Value = timeNow;
string query = @"SELECT s.TagID, se.SessionID, '" +
DateTime.Now.ToString("MM/dd/yy HH:mm:ss tt") +
"' AS ScanningTime " +
" FROM (((Student s " +
" LEFT JOIN [CourseID-ModuleID] cm ON s.CourseID = cm.CourseID) " +
" LEFT JOIN [ModuleID-SessionID] ms ON ms.ModuleID = cm.ModuleID) " +
" LEFT JOIN [Session] se ON ms.SessionID = se.SessionID) " +
" WHERE s.TagID = @tagNo " +
" AND se.SessionDate = Date() " +
" AND @timeNow >= DATE_SUB(SessionTimeStart,INTERVAL -30 MINUTE) " +
" AND se.SessionTimeEnd >= @Plus30Min ";
时间是以“18:00 PM”格式保存的 DB。删除该行后我可以成功运行查询，这是我遇到的问题。
" AND @timeNow >= DATE_SUB(SessionTimeStart,INTERVAL -30 MINUTE) " +
我的目标是用这样的东西替换上面的行：
"AND CurrentTime >= se.SessionTimeStart - 30 minutes "
我的完整 C# 代码：
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Data.OleDb;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO.Ports;
namespace AutoReg
{
public partial class Form1 : Form
{
// Create the serial port with basic settings
public SerialPort port = new SerialPort("COM3", 9600, Parity.None, 8, StopBits.One);
public int tagNo;
public Form1()
{
InitializeComponent();
//Attach a method to be called when there is data waiting in the port's buffer
port.DataReceived += new SerialDataReceivedEventHandler(RFID_DataReceived);
//Begin communications
port.Open();
}
public void setSQL()
{
string ConnStr = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\Users\\Kacper\\Desktop\\AutoReg\\AutoReg\\AutoReg.accdb;";
//DateTime TimeMinus = DateTime.Now.AddMinutes(-30);
DateTime TimePlus = DateTime.Now.AddMinutes(30);
DateTime now = DateTime.Now;
//string Minus30Min = TimeMinus.ToString("hh:mm tt");
string Plus30Min = TimePlus.ToString("hh:mm tt");
string timeNow = now.ToString("hh:mm tt");
OleDbConnection MyConn = new OleDbConnection(ConnStr);
MyConn.Open();
DataSet ds = new DataSet();
//SQL query that finds the current sessionID for the given tagID
string query = @"SELECT s.TagID, se.SessionID, '" +
DateTime.Now.ToString("MM/dd/yy HH:mm:ss tt") +
"' AS ScanningTime " +
" FROM (((Student s " +
" LEFT JOIN [CourseID-ModuleID] cm ON s.CourseID = cm.CourseID) " +
" LEFT JOIN [ModuleID-SessionID] ms ON ms.ModuleID = cm.ModuleID) " +
" LEFT JOIN [Session] se ON ms.SessionID = se.SessionID) " +
" WHERE s.TagID = @tagNo " +
" AND se.SessionDate = Date() " +
" AND @timeNow >= DATEADD(MI,-30,se.SessionTimeStart) " +
" AND se.SessionTimeEnd >= @Plus30Min ";
OleDbCommand command = new OleDbCommand(query, MyConn);
command.Parameters.Add("tagNo", OleDbType.Integer).Value = tagNo;
//command.Parameters.Add("Minus30Min", OleDbType.VarChar).Value = Minus30Min;
command.Parameters.Add("Plus30Min", OleDbType.VarChar).Value = Plus30Min;
command.Parameters.Add("timeNow", OleDbType.DBTime).Value = timeNow;
OleDbDataAdapter adapter = new OleDbDataAdapter(command);
adapter.Fill(ds);
dataGridView3.DataSource = ds.Tables[0];
MyConn.Close();
}
public void RFID_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
while (port.ReadChar() != 2) ;
int v = 0;
port.ReadChar(); // drop 1st 2 bytes - we actually only read the lower 32-bits of the code
port.ReadChar();
for (int i = 7; i >= 0; i--)
{
int c = port.ReadChar(); // a ascii hex char
int part = c - '0';
// test if 'Alpha'
if (part > 9) part -= 7; // Quick & dirty !
v |= part << (i * 4);
}
for (int i = 0; i < 5; i++)
{
port.ReadChar();
}
tagNo = v; ;
this.Invoke(new MethodInvoker(delegate()
{
//if the scanned tag already exists in the Student table...
var foundIDs = autoRegDataSet.Student.Select("TagID = '" + tagNo + "'");
if (foundIDs.Length != 0)
//if (tagNo == autoRegDataSet.Student[0][0].ToString())
{
textBox1.Text = "Tag in database, tagNO: " + v.ToString();
DataRow row = autoRegDataSet.Attendance.Rows.Add();
row[0] = v.ToString();
row[2] = DateTime.Now;
}
else
{
textBox1.Text = "NOT in database, tagNO: " + v.ToString();
}
}));
}
private void Form1_Load(object sender, EventArgs e)
{
// TODO: This line of code loads data into the 'autoRegDataSet.Attendance' table. You can move, or remove it, as needed.
this.attendanceTableAdapter.Fill(this.autoRegDataSet.Attendance);
// TODO: This line of code loads data into the 'autoRegDataSet.ActiveSessions' table. You can move, or remove it, as needed.
this.activeSessionsTableAdapter.Fill(this.autoRegDataSet.ActiveSessions);
// TODO: This line of code loads data into the 'autoRegDataSet.Student' table. You can move, or remove it, as needed.
this.studentTableAdapter.Fill(this.autoRegDataSet.Student);
}
private void button1_Click(object sender, EventArgs e)
{
setSQL();
}
}
}