Naming Column Header Based On Results From Database - vb.net

Question

net and would to have the Header Text of columns in a datagridview be named after results from the database, e.g the query in my code returns four dates,30/08/2017,04/09/2017,21/09/2017 and 03/02/2018. My aim is to have the column headers in the data grid named after those dates. Your help will highly be appreciated.
sql = "SELECT COUNT (ServiceDate) As NoOfServiceDates FROM (SELECT DISTINCT ServiceDate FROM tblattendance)"
Using command = New OleDbCommand(sql, connection)
Using reader = command.ExecuteReader
reader.Read()
ColumnNo = CInt(reader("NoOfServiceDates")).ToString
End Using
End Using
DataGridView1.ColumnCount = ColumnNo
For i = 0 To DataGridView1.Columns.Count - 1
sql = "SELECT DISTINCT ServiceDate FROM tblattendance"
Using command = New OleDbCommand(sql, connection)
Using reader = command.ExecuteReader
While reader.Read
DataGridView1.Columns(i).HeaderText = reader("ServiceDate").ToString
End While
End Using
End Using
Next

Answer 1

The current code re-runs the query each time through the column count loop, meaning it will set the column header for that column to all of the date values in sequence, so the last value in the query shows in the all the columns. You only need to run the query once:
Dim i As Integer = 0
sql = "SELECT DISTINCT ServiceDate FROM tblattendance"
Using command As New OleDbCommand(sql, connection), _
reader As OleDbDatareader = command.ExecuteReader()
While reader.Read
DataGridView1.Columns(i).HeaderText = reader("ServiceDate").ToString
i+= 1
End While
End Using
Additionally, this still results in two separate trips to the database, where you go once to get the count and again to get the values. Not only is this very bad for performance, it leaves you open to a bug where another user changes your data from one query to the next.
There are several ways you can get this down to one trip to the database: loading the results into memory via a List or DataTable, changing the SQL to include the count and the values together, or adding a new column each time through the list. Here's an example using the last option:
DataGridView1.Columns.Clear()
Dim sql As String = "SELECT DISTINCT ServiceDate FROM tblattendance"
Using connection As New OleDbConnection("string here"), _
command As New OleDbCommand(sql, connection)
connection.Open()
Using reader As OleDbDataReader = command.ExecuteReader()
While reader.Read
Dim column As String = reader("ServiceDate").ToString()
DataGridView1.Columns.Add(column, column)
End While
End Using
End Using
Even better if you can use something like Sql Server's PIVOT keyword in combination with the DataGridView's AutoGenerateColumns feature for DataBinding, where you will write ONE SQL statement that has both column info and data, and simply bind the result set to the grid.

Answer 2

The For Next is incorrect. You execute your command for every column, when you only need to execute it once. The last result from the DataReader will be the header for every column as currently written.
You should iterate through your DataReader and increment the cursor variable there:
Dim i As Integer = 0
Using command = New OleDbCommand(sql, connection)
Using reader = command.ExecuteReader
While reader.Read
DataGridView1.Columns(i).HeaderText = reader("ServiceDate").ToString
i += 1
End While
End Using
End Using