Access VBA: Calculating Median on data using GROUP BY on two columns - sql

Question

I am trying to find a way to calculate the median of a dataset in access, that is grouped by two columns, typeA, typeB.
This is a sample of the table:
ID (autonumber)
typeA (large number)
typeB (large number)
total (large number)
1
1
1
15
2
2
1
15
3
1
1
45
4
2
1
44
5
1
2
19
6
1
2
4
7
1
2
34
8
2
2
19
9
2
2
18
Using Access 2016
Currently I am using the following code snippet:
Function fMedian(SQLOrTable, GroupFieldName, GroupFieldValue, GroupFieldName2, GroupFieldValue2, MedianFieldName)
DoCmd.SetWarnings False
Dim rs As DAO.Recordset
Set db = CurrentDb
Set rs1 = db.OpenRecordset(SQLOrTable, dbOpenDynaset)
If IsDate(GroupFieldValue) Then
GroupFieldValue = "#" & GroupFieldValue & "#"
ElseIf Not IsNumeric(GroupFieldValue) Then
GroupFieldValue = "'" & Replace(GroupFieldValue, "'", "''") & "'"
End If
If IsDate(GroupFieldValue2) Then
GroupFieldValue2 = "#" & GroupFieldValue2 & "#"
ElseIf Not IsNumeric(GroupFieldValue) Then
GroupFieldValue2 = "'" & Replace(GroupFieldValue2, "'", "''") & "'"
End If
rs1.Filter = GroupFieldName & "=" & GroupFieldValue
rs1.Sort = MedianFieldName
Set rs = rs1.OpenRecordset()
rs.Move (rs.RecordCount / 2)
If rs.RecordCount Mod 2 = 0 Then
varMedian1 = rs.Fields(MedianFieldName)
rs.MoveNext
fMedian = varMedian1 + rs.Fields(MedianFieldName) / 2
Else
fMedian = rs.Fields(MedianFieldName)
End If
End Function
As it stands, this works great for grouping by one column, but I cannot figure out how to allow it to group by on both typeA and typeB. I have by editing the rs1.filter line but to no avail.
Any help with the code, or a better approach would be appreciated.
Thank you!
NOTE: solved using parfaits solution below. added line medianVBA = fmedian before the end of the function.

Answer 1

Consider an extension of #Fionnuala's great answer to calculate median in MS Access by accommodating an open-ended number of grouping variables.
VBA (save below in a standard module of Access project)
Code builds a dynamic SQL string for DAO recordset call for later median calculation. Special handling required for groupings with 0-2 records and null values for groupings.
Public Function MedianVBA(ParamArray Arr() As Variant) As Double
On Error GoTo ErrHandle
Dim N As Long
Dim tblName As String, numCol As String, grpVals As String
Dim strSQL As String
Dim db As DAO.Database, rs As DAO.Recordset
Dim varMedian As Double, fMedian As Double
'BUILD DYNAMIC SQL
tblName = Arr(0)
numCol = Arr(1)
grpVals = " WHERE " & numCol & " IS NOT NULL "
For N = 2 To UBound(Arr) Step 2
If Arr(N + 1) = "" Or IsNull(Arr(N + 1)) Then
grpVals = grpVals & " AND " & Arr(N) & " IS NULL"
ElseIf IsDate(Arr(N + 1)) Then
grpVals = grpVals & " AND " & Arr(N) & " = #" & Arr(N + 1) & "#"
Else
grpVals = grpVals & " AND CStr(" & Arr(N) & ") = '" & Arr(N + 1) & "'"
End If
Next N
strSQL = "SELECT " & numCol _
& " FROM " & tblName _
& grpVals _
& " ORDER BY " & numCol
'CALCULATE MEDIAN
Set db = CurrentDb
Set rs = db.OpenRecordset(strSQL, dbOpenDynaset)
If rs.RecordCount = 0 Then
MedianAcc = fMedian
GoTo ExitHandle
ElseIf rs.RecordCount = 1 Then
MedianAcc = rs.Fields(numCol)
GoTo ExitHandle
End If
rs.Move (rs.RecordCount / 2)
rs.MovePrevious
If rs.RecordCount Mod 2 = 0 Then
varMedian = rs.Fields(numCol)
If rs.RecordCount = 2 Then
rs.MoveLast
Else
rs.MoveNext
End If
fMedian = (varMedian + rs.Fields(numCol)) / 2
Else
fMedian = rs.Fields(numCol)
End If
rs.Close
MedianAcc = fMedian
ExitHandle:
Set rs = Nothing: Set db = Nothing
Exit Function
ErrHandle:
MsgBox Err.Number & ": " & Err.Description, vbCritical, "RUNTIME ERROR"
Resume ExitHandle
End Function
Do note, above VBA function uses a ParamArray where first argument expects the source table and second column expects the numeric column and the remaining is open-ended for group column name and value pairs. Signature of call is as follows:
=MedianAcc("table_name",
"numeric_col",
"group1_column", "group1_value",
"group2_column", "group2_value",
...)
SQL (stored query that calls above VBA function)
Below runs a one-group and two-group median calculation.
SELECT t.typeA, t.typeB
, MedianVBA('[myTable]', '[total]', '[typeA]', t.typeA) AS MedianGrp1,
, MedianVBA('[myTable]', '[total]', '[typeA]', t.typeA, '[typeB]', t.typeB) AS MedianGrp2
FROM myTable t
GROUP BY t.typeA, t.typeB

Answer 2

Excuse me taking a totally different approach here...
Say you have a Table called Table1 with fields Field1.
To find the median of Field1 the SQL query would look like this:
SELECT TOP 1
((SELECT MAX(B.Field1) AS Field1 FROM
(SELECT TOP 50 PERCENT A.Field1 FROM Table1 A)
B) +
(SELECT MIN(D.Field1) AS Field1 FROM
(SELECT TOP 50 PERCENT C.Field1 FROM Table1 C ORDER BY C.Field1 DESC)
D))
/2 AS MEDIAN FROM Table1
(the above split out to make it more readable, I wrote it as only 2 lines)
From there all you have to do is write the vba to make it dynamic - replace 'Table1' with your variable SQLorTable and Field1 with the field to find the median of.