Use public function in sql string - vb.net

Question

I have a project in vb.net that uses a mysql backend. In my project I have a module that contains a number of public functions that I call from different forms. Now I am trying to get data from my db to populate a grid but one of the columns needs to be calculated using one of my public functions. How do I call the function from my SQL string to get the data for that particular column in the grid. I'm getting the data from the db into a datatable and binding it to my grid.
--Edit---
This is the code of the function my want to call from my select query:
Public Function getCurrentQuantity(ByRef ItemID As Integer) As Double
Dim conn As MySqlConnection = New MySqlConnection(gen.connString)
Dim result As Double
Dim recValue As Double 'quantity received
Dim issValue As Double 'quantity issued
Dim adjValueNeg As Double 'negative adjustment of quantity on hand
Dim adjValuePos As Double 'positive adjustment of quantity on hand
Dim p_retValue As Double 'quantity returned to suppliers
Dim i_retValue As Double 'quantity returned to stores
Dim openValue As Double ' opening qty
Dim countLast As Integer = doCount("ID", "inventory_current", " WHERE Item = " & ItemID)
Dim stockDate As Date 'stock date is used to determine where to pick the opening balance from
Dim hasLastStockBal As Boolean 'to determine if the item has a last stock balance
Dim tranTypes As New List(Of String) 'creating a list of the different transaction types to loop through them
tranTypes.AddRange({"RECV", "ISSU", "ADJN", "ADJP", "RETN", "RETI", "OBAL"})
Try
If conn.State <> ConnectionState.Open Then
conn.Open()
End If
If countLast = 0 Then 'no last inventory so opening bal will be from the item table
hasLastStockBal = False
Else
hasLastStockBal = True
End If
For i = 0 To 6 'loop through the list of transaction types
sql = "SELECT SUM(Quantity) FROM stock_dairy WHERE EntryDate > #StockDate AND Item = #Item AND EntryType = #Type"
cmd = New MySqlCommand(sql, conn)
cmd.Parameters.AddWithValue("#StockDate", stockDate)
cmd.Parameters.AddWithValue("#Item", ItemID)
cmd.Parameters.AddWithValue("#Type", tranTypes(i))
da.SelectCommand = cmd
da.Fill(ds, "dat")
rowscount = ds.Tables("dat").Rows.Count
If rowscount = 0 Then
result = 0
Else
If IsDBNull(ds.Tables("dat").Rows(0).Item(0)) Then
result = 0
Else
result = ds.Tables("dat").Rows(0).Item(0)
End If
End If
If i = 0 Then 'assign the retrieved variable based on the For..Next position
recValue = result
ElseIf i = 1 Then
issValue = result
ElseIf i = 2 Then
adjValueNeg = result
ElseIf i = 3 Then
adjValuePos = result
ElseIf i = 4 Then
p_retValue = result
ElseIf i = 5 Then
i_retValue = result
ElseIf i = 6 Then
If hasLastStockBal = True Then
openValue = dLookup("Quantity", "inventory_current", " WHERE Item = " & ItemID)
Else
openValue = result
End If
End If
Next
result = openValue + (((recValue - p_retValue) + adjValuePos) - ((issValue - i_retValue) + adjValueNeg)) 'sum up and return
Return result
Catch ex As Exception
MsgBox(ex.Message, vbExclamation, "Unexpected Error")
Return 0
Finally
ds.Reset()
conn.Dispose()
End Try
End Function