VB.NET/MS ACCESS 显示查询数据

Question

I have a textbox that I need to run this query when the value changes for the textbox.

Private Sub tbStudentID2_TextChanged(sender As Object, e As EventArgs) Handles tbStudentID2.TextChanged
    mySql = "SELECT DateDiff('d', [DateofRank], [currentDate]) FROM StudentData"
    ConnDB()
    myCommand = New OleDbCommand(mySql, myConnection)
    myCommand.ExecuteNonQuery()
    myRead = myCommand.ExecuteReader
    Do While myRead.Read = True
        tbDays.Text = myRead(0).ToString()
    Loop
End Sub

86 is display no matter which record I choose. I think it is just retrieving the first record in the in the table.

I want to display the number of days since a student has tested by subtracting DateofRank from currentDate. I am open to other ideas on how to achieve this.

Answer 1

You're overwriting the value of the TextBox with each pass. Make sure Multiline is set to true and try the following.

Dim builder As New StringBuilder
Do While myRead.Read = True
    'tbDays.Text &= myRead(0).ToString()  Note the &= to keep concatenating the string.
    builder.Append(myRead(0).ToString())
Loop
tbDays.Text = builder.ToString()

Answer 2

I think you just need to introduce a condition ~ WHERE ID = tbStudentID2.Text

Private Sub tbStudentID2_TextChanged(sender As Object, e As EventArgs) Handles tbStudentID2.TextChanged
    Dim id As Integer
    If Integer.TryParse(tbStudentID2.Text, id) Then
        mySql = $"SELECT DateDiff('d', [DateofRank], [currentDate]) FROM StudentData WHERE id={id}"
        ConnDB()
        myCommand = New OleDbCommand(mySql, myConnection)
        myCommand.ExecuteNonQuery()
        myRead = myCommand.ExecuteReader()
        Do While myRead.Read()
            tbDays.Text = myRead(0).ToString()
        Loop
    End If
End Sub

I guess your ID is an Integer. If that's the case, this will work, and is immune to SQL injection. I validated your input. Do you want to query when a typo happens? Also, do you really want to query, when typing 12345, ID=1, ID=12, ID=123, ID=1234, and ID=12345? Maybe it's not typed in - then it's OK.

You may need to change the condition to the appropriate field name ie [StudentData].[ID] ?

Also, this will always result in the last match in the table. If you don't expect more than one match, then you can just take the first. You could eliminate the Do While loop.

I see some other issues such as disposable objects not being disposed. You shouldn't keep your database connection open when you aren't using it. The objects which should be disposed of immediately, in order, are myRead , myCommand , and myConnection . This is easily handled by declaring them with the Using keyword, in the reverse order of my list. ConnDB() could return your connection in that case.

Answer 3

This is how I fixed with help from a post on an another site. I do have one question though. If I comment out the first db connection - ConnDB() I get an error. Why does it need two connection to work? It makes a connection in the using command. I had setup a data table from a class I wrote to make a connection and set it up so I declare Private Access As New DBControl at the top and I can add parameters and do my additions and edits that way. I didn't know how to make that work with the existing connection to the Access DBControl so I just did another connection. Not the best but I am a beginner and don't know how to do things with more efficiency yet.

Private Sub tbStudentID2_TextChanged(sender As Object, e As EventArgs) Handles tbStudentID2.TextChanged
    mySql = "SELECT DateDiff('d', [DateofRank], [currentDate]) FROM StudentData"
    '"SELECT DateDiff('d', [DateofRank], [currentDate]) FROM StudentData WHERE id={id}"
    ConnDB()
    'myCommand = New OleDbCommand(mySql, myConnection)
    'myCommand.ExecuteNonQuery()

    Dim studentID As Integer
    If Not Integer.TryParse(tbStudentID2.Text, studentID) Then
        ' Not a valid ID
        tbDays.Text = String.Empty
    Else
        Using command As New OleDbCommand("SELECT DateDiff('d', [DateofRank], [currentDate]) FROM StudentData WHERE StudentID = @StudentID", myConnection)
            command.Parameters.AddWithValue("@StudentID", studentID)

            ConnDB()

            Dim days As Object = command.ExecuteScalar()
            tbDays.Text = If(Convert.IsDBNull(days), String.Empty, Convert.ToString(days))
        End Using


        ' ADD PARAMETERS - ORDER MATTERS !!!
        Access.AddParam("@TIG", tbDays.Text)
        Access.AddParam("@SID", tbStudentID2.Text)

        ' RUN COMMAND
        Access.ExecQuery("UPDATE StudentData " &
                         "SET TimeinGrade=@TIG " &
                         "WHERE StudentID=@SID")

        ' REPORT & ABORT ON ERRORS
        If NoErrors(True) = False Then Exit Sub
    End If
End Sub