Incorrect syntax near ',' (dbnull issue) - asp.net

Question

It suppose there is chart to be appeared. But, it doesnt as there is problem regarding dbnull issue. This happen when either one of three select statement has no data.
Dim user As String = Session("NoMatrik")
Dim resultId As Object = Session("max")
Dim idQuery = "select max(resultid) as id from tblResult where result_nomatric = #matric and result_quiz_id = 1 UNION All " +
"select max(resultid) as id from tblResult where result_nomatric = #matric and result_quiz_id = 2 UNION All " +
"select max(resultid) as id from tblResult where result_nomatric = #matric and result_quiz_id = 3"
conn.Open()
Dim cmdGetId As New SqlCommand(idQuery, conn)
cmdGetId.Parameters.AddWithValue("#matric", user)
Dim maxIDs As SqlDataReader = cmdGetId.ExecuteReader
Dim IDs As String = ""
While maxIDs.Read
IDs += maxIDs("id").ToString() + ", "
End While
maxIDs.Close()
IDs = IDs.Substring(0, IDs.Length - 2)
Dim cmdString = "Select tblResult.result_quiz_id as Quiz,count(TblAnswer.AnswerType) as answerCount , TblAnswer.AnswerType " +
"from TblResultDetail inner join TblAnswer on TblResultDetail.ResultDetail_Answer_Id = TblAnswer.AnswerId " +
"inner join tblResult on tblResult.resultid = TblResultDetail.ResultDetail_Result_Id " +
"where TblResultDetail.ResultDetail_Result_Id in (" + IDs + ") " +
"group by TblAnswer.AnswerType, tblResult.result_quiz_id order by TblAnswer.AnswerType"
Dim cmd As New SqlCommand(cmdString, conn)
If IsDBNull(resultId) Then
Label1.Visible = True
chrtResult.Visible = False
Else
Dim dr1 As SqlDataReader
dr1 = cmd.ExecuteReader
While dr1.Read()
Dim tempArr(0) As Double
Dim count As Double = dr1("answerCount")
tempArr(0) = count
Dim Type As String = dr1("AnswerType").ToString()
Dim level As Integer = dr1("Quiz")
chrtResult.Series(Type).Points(level - 1).YValues = tempArr
End While
End If
conn.Close()
End If
End Sub
an error, Incorrect syntax near '(' appear at line dr1 = cmd.ExecuteReader. So , how I want to fix this error?

Answer 1

Update your first query to exclude any Null values using a HAVING clause like so:
Dim idQuery = "select max(resultid) as id from tblResult " +
"where result_nomatric = #matric and result_quiz_id = 1 " +
"having max(resultid) is not null " +
"UNION All " +
"select max(resultid) as id from tblResult " +
"where result_nomatric = #matric and result_quiz_id = 2 " +
"having max(resultid) is not null " +
"UNION All " +
"select max(resultid) as id from tblResult " +
"where result_nomatric = #matric and result_quiz_id = 3 " +
"having max(resultid) is not null"
The having max(resultid) is not null will exclude any nulls in your UNION ALL.
If there are no IDs returned, you simply need to do a check on this before you execute your next block of code and do as #DmitriE suggests with the adding of quotes. Reorganise it to look like:
While maxIDs.Read
IDs += "'" + maxIDs("id").ToString() + "', "
End While
If IDs = "" Then
Label1.Visible = True
chrtResult.Visible = False
Else
IDs = IDs.Substring(0, IDs.Length - 2)
Dim cmdString = "Select ....."
Dim dr1 As SqlDataReader
dr1 = cmd.ExecuteReader
While dr1.Read()
' YOUR WHILE LOOP CODE HERE'
End While
End If

Answer 2

This should be
While maxIDs.Read
IDs += "'" + maxIDs("id").ToString() + "', "
End While
and then you need to remove last single quote.
in clause should follow this format: where x.id in ('id1', 'id2', 'idN')