how can i open execuete a query in VB while there is reader opened? - mysql

Question

is there any possible way to execute this without getting this error "There is already an open DataReader associated with this Connection which must be closed first." i already tried using "dr.close()" and i get another error that says "Invalid attempt to Read when reader is closed." can you help me out?
Heres my code:
Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click
Label2.Text = AllPicker1.Text
Label3.Text = AllPicker2.Text
If AllPicker1.Value >= AllPicker2.Value Then
MsgBox("End Date Must be Greater!")
Else
Dim SQLstatement As String = "SELECT * FROM tblStudInfo,tbl_studentLog WHERE tblStudInfo.StudID = tbl_studentLog.StudentNumber AND tbl_studentLog.LoginDate BETWEEN '" & AllPicker1.Text & "' AND '" & AllPicker2.Text & "'"
OpenData(SQLstatement)
End If
End Sub
Public Sub OpenData(ByRef SQLstatement As String)
Dim cmd As MySqlCommand = New MySqlCommand
With cmd
.CommandText = SQLstatement
.CommandType = CommandType.Text
.Connection = SqlConnection
dr = .ExecuteReader()
End With
While dr.Read
Dim SQLstatementSave As String = "INSERT INTO tbl_report (RepStudNo,RepName,RepCourse,RepDept,RepLogTime,RepLogdate) VALUES ('" & dr("StudID") & "','" & dr("Name") & "','" & dr("Course") & "','" & dr("Dept") & "','" & dr("LoginTime") & "','" & dr("LoginDate") & "') "
dr.Close()
Save(SQLstatementSave)
End While
SqlConnection.Close()
SqlConnection.Dispose()
SqlConnection.Open()
End Sub
Public Sub Save(ByRef SQLstatementSave As String)
Dim cmd As MySqlCommand = New MySqlCommand
With cmd
.CommandText = SQLstatementSave
.CommandType = CommandType.Text
.Connection = SqlConnection
.ExecuteNonQuery()
End With
SqlConnection.Close()
SqlConnection.Dispose()
SqlConnection.Open()
End Sub
End Class

Answer 1

It seems you are using only one SqlConnection. For most database systems you cannot reuse the connection while you are reading from it. You can either read all data into memory / DataTable and work on the rows after that or use a different SqlConnection for your Inserts.
When working with SqlConnections, Readers and Commands I find the Using Statement very helpful to visualize object usage and creation.

Answer 2

We can reduce this down to a single query:
INSERT INTO tbl_report
(RepStudNo,RepName,RepCourse,RepDept,RepLogTime,RepLogdate)
SELECT StudID, Name, Course, Dept, LoginTime, LoginDate
FROM tblStudInfo
INNER JOIN tbl_studentLog ON tblStudInfo.StudID = tbl_studentLog.StudentNumber
WHERE tbl_studentLog.LoginDate BETWEEN #StartDate AND #EndDate
Note the use of the full INNER JOIN syntax. The older TableA,TableB syntax for joins should be avoided. Also note the use of placeholders for your dates. This is important.
Now I need to draw attention to a couple functions I saw: OpenData(), and Save().
Those two functions are fundamentally broken, because they force you to build your queries in a way that leaves you vulnerable to sql injection hacking. Someday soon, someone will put a value like this into a textbox that is included with a query:
';DROP Table tbl_studentLog;--
Think carefully about what would happen now if someone entered that into your AllPicker1.Text. It would be hard to do that to a date picker, but I'll bet you have other plain text fields that would allow this. The first character (single quote) in my proposed input would close the string literal in the query. The second character (semi-colon) would end the individual statement, but sql server won't stop executing code. The next set of characters make up an additional statement that would drop your table. The final two characters comment out anything that follows, to avoid sql server rejecting or not committing the command because of syntax errors. Yes, Sql Server will run that additional statement, if that is what you put in a textbox.
So, your methods as written are broken, because the only accept completed sql strings as input. Any function that calls into the database MUST also include a mechanism for accepting query parameters. You ultimately want to be running code more like this:
Public Sub CreateReport(ByVal StartDate As DateTime, ByVal EndDate As DateTime)
Dim sql As String = _
"INSERT INTO tbl_report " & _
" (RepStudNo,RepName,RepCourse,RepDept,RepLogTime,RepLogdate) " & _
" SELECT StudID, Name, Course, Dept, LoginTime, LoginDate " & _
" FROM tblStudInfo " & _
" INNER JOIN tbl_studentLog ON tblStudInfo.StudID = tbl_studentLog.StudentNumber " & _
" WHERE tbl_studentLog.LoginDate BETWEEN #StartDate AND #EndDate"
'.Net is designed such in most cases that you really do want a new SqlConnection for each query
'I know it's counter-intuitive, but it is the right way to do this
Using cn As New SqlConnection("Connection string"), _
cmd As New SqlCommand(sql, cn)
'Putting your data into the query using parameters like this is safe from injection attacks
cmd.Parameters.Add("#StartDate", SqlDbType.DateTime).Value = StartDate
cmd.Parameters.Add("#EndDate", SqlDbType.DateTime).Value = EndDate
cn.Open()
cmd.ExecuteNonQuery()
End Using
End Sub
One thing to point out here is that at first glance I don't close the connection. However, the Using block will ensure that the connection is closed promptly... even if an exception is thrown. Your existing code will leave the connection hanging in the case of a exception.
Also note that this neatly side-steps the whole issue of needing to execute a separate query while your reader is opened... but if you ever do really need to do this (it's rare), the answer is simple: use a separate connection.

Answer 3

Instead of:
Dim SQLstatementSave As String = "INSERT INTO tbl_report
(RepStudNo,RepName,RepCourse,RepDept,RepLogTime,RepLogdate)
VALUES ('" & dr("StudID") & "','" & etc.
Try using .ToString on your DR() references.
Dim SQLstatementSave As String = "INSERT INTO tbl_report
(RepStudNo,RepName,RepCourse,RepDept,RepLogTime,RepLogdate)
VALUES ('" & dr("StudID").ToString & "','" & etc.