How to detect it was duplicate entry on VB.NET? - sql-server

Question

I have a problem on making avoid or detect duplicate data entry in VB.NET using SQL Server, and also pop up message when there is duplicate entry.
This is my add button code:
Dim sql = "INSERT INTO studentAttendence(studentID, date, time, subject, attendence) " &
"VALUES(#studentID, #date, #time, #subject, #attendence)" &
"WHERE NOT EXISTS(SELECT * FROM studentAttendence WHERE studentID = #studentID)"
Using cn As New SqlConnection("My sql Connection string")
Dim commands As SqlCommand = New SqlCommand(sql, cn)
commands.Parameters.Add("#studentID", SqlDbType.NVarChar).Value = TextBox1.Text
commands.Parameters.Add("#date", SqlDbType.NVarChar).Value = DateTimePicker1.Text
commands.Parameters.Add("#time", SqlDbType.NVarChar).Value = DateTimePicker2.Text
commands.Parameters.Add("#subject", SqlDbType.NVarChar).Value = ComboBox1.Text
commands.Parameters.Add("#attendence", SqlDbType.NVarChar).Value = ComboBox2.Text
commands.ExecuteNonQuery()
End Using
Or is there any other methods for making avoid duplicate entries. I just want to avoid same date and subject of a student to add. So I can only add subject on the next date if I already add subject on today's date of studentID.

Answer 1

You can use If Not Exists before the insert. This method only requires a single hit on the database. Add a comman at the end of the first line of the Using block, then delete Dim before commands. This will include commands in the Using block.
Private Function InsertAttendance() As Integer
Dim sql = "If Not Exists (Select 1 From studentAttendence
Where studnetID = #studentID And date = #date And subject = #subject)
INSERT INTO studentAttendence(studentID, date, time, subject, attendence)
VALUES(#studentID, #date, #time, #subject, #attendence);"
Dim NumRowsAffected As Integer
Using cn As New SqlConnection("My sql Connection string"),
commands As SqlCommand = New SqlCommand(sql, cn)
commands.Parameters.Add("#studentID", SqlDbType.NVarChar).Value = TextBox1.Text
commands.Parameters.Add("#date", SqlDbType.NVarChar).Value = DateTimePicker1.Text
commands.Parameters.Add("#time", SqlDbType.NVarChar).Value = DateTimePicker2.Text
commands.Parameters.Add("#subject", SqlDbType.NVarChar).Value = ComboBox1.Text
commands.Parameters.Add("#attendence", SqlDbType.NVarChar).Value = ComboBox2.Text
cn.Open()
NumRowsAffected = commands.ExecuteNonQuery()
End Using
Return NumRowsAffected
End Function
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Try
If InsertAttendance() = 1 Then
MessageBox.Show("Success")
Else
MessageBox.Show("Duplicate Entry")
End If
Catch ex As Exception
MessageBox.Show($"Error Entering Attendance {ex.Message}")
End Try
End Sub
Be sure to check the datatypes of your parameters with the database.

Answer 2

You should create a function that queries your data table to return the row count where the Date, Subject, and StudentId match what you're wanting to insert. If the returned result is greater than 0 then display the message.
Here is a function that returns -1 if something went wrong or the row count:
Private Function GetRowCount(ByVal studentIdParameter As String, ByVal dateParameter As DateTime, ByVal subjectParameter As String) As Integer
'Declare the object to return
Dim count As Integer = -1
'Declare the connection object
Dim con As OleDbConnection
'Wrap code in Try/Catch
Try
'Set the connection object to a new instance
'TODO: Change "My Connection String Here" with a valid connection string
con = New OleDbConnection("My Connection String Here")
'Create a new instance of the command object
'TODO: Change [ID] to a valid column
Using cmd As OleDbCommand = New OleDbCommand("SELECT Count([ID]) FROM [MyTable] WHERE studentID=#studentId, AND date=#date AND subject=#subject", con)
'Parameterize the query
cmd.Parameters.Add("#studentId", OleDbType.NVarChar).Value = studentIdParameter
cmd.Parameters.Add("#date", OleDbType.DBDate).Value = dateParameter.Date
cmd.Parameters.Add("#subject", OleDbType.NVarChar).Value = subjectParameter
'Open the connection
con.Open()
'Use ExecuteScalar to return a single value
count = Convert.ToInt32(cmd.ExecuteScalar())
'Close the connection
con.Close()
End Using
Catch ex As Exception
'Display the error
Console.WriteLine(ex.Message)
Finally
'Check if the connection object was initialized
If con IsNot Nothing Then
If con.State = ConnectionState.Open Then
'Close the connection if it was left open(exception thrown)
con.Close()
End If
'Dispose of the connection object
con.Dispose()
End If
End Try
'Return the row count
Return count
End Function
To implement it, you'd do something like:
Dim rowCount = GetRowCount(TextBox1.Text, DateTimePicker1.Value, ComboBox1.Text)
If (rowCount = -1) Then
MessageBox.Show("Something went wrong checking for duplicates")
ElseIf (rowCount = 0) Then
' Insert record
Else
MessageBox.Show("A record already exists with this studentId, date, and subject.")
End If