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ACC2000: How to Create a "Top N Values Per Group" Query

Article ID: 210039 - View products that this article applies to.

This article was previously published under Q210039

Moderate: Requires basic macro, coding, and interoperability skills.

This article applies only to a Microsoft Access database (.mdb).

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SUMMARY

This article shows you two methods that you can use to create queries that list only the top n items per group. By using either one of these methods, you can create a query that displays only the top five salespeople for each region.

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MORE INFORMATION

To list only the top n items within a group in a query, you must specify a criteria that dynamically reads the grouping column in the query and limits the item column to the top n values within each group.

Method 1 uses a SQL subquery to dynamically generate a list of the top n items for each group, and then uses this list as the criteria for the item column by using the In operator.

Method 2 uses a user-defined function to return the nth item within a specific group, which is then used with the >= operator to return the nth-and-greater items.

Method 1

Follow these steps to create a query in the Northwind sample database that displays the top three UnitsInStock per CategoryID. The query uses a SQL subquery, which returns the top three UnitsInStock, given a specific CategoryID, and then uses the In operator to limit the records in the main query.

NOTE: In the criteria example in step 5, an underscore (_) at the end of a line is used as a line-continuation character. Remove the underscore from the end of the line when re-creating the criteria.

CAUTION: If you follow the steps in this example, you modify the sample database Northwind.mdb. You may want to back up the Northwind.mdb file and follow these steps on a copy of the database.

Open the sample database Northwind.mdb.
Click Queries, and then click New.
Click Design View, and then click OK.
In the Show Table dialog box, click Categories and Products, click Add, and then click Close.
Add the following fields to the query grid:
Field: CategoryName
Sort: Ascending

Field: ProductName

Field: UnitsInStock
Sort: Descending
Criteria: In (Select Top 3 [UnitsInStock] From Products Where _
[CategoryID]=[Categories].[CategoryID] Order By [UnitsInStock] Desc)
Run the query.

Note that the query returns the top three UnitsInStock for each category.

Method 2

Follow these steps to create a query in the Northwind sample database that displays the last five orders placed per customer. The query uses a criteria function called NthInGroup() that returns the fifth most recent OrderDate, given a specific CustomerID. If you use the >= operator along with this function, all orders that were placed on or after the fifth most recent OrderDate for each customer will be returned.

CAUTION: If you follow the steps in this example, you modify the sample database Northwind.mdb. You may want to back up the Northwind.mdb file and follow these steps on a copy of the database.

Open the sample database Northwind.mdb.
Create a new module, and then add the following line to the Declarations section if it is not already there:
Option Explicit

Create the following procedure:

Function NthInGroup(GroupID, N)
' Returns the Nth Item in GroupID for use as a Top N per group
' query criteria.
Static LastGroupId, LastNthInGroup
Dim ItemName, GroupIDName, GDC, SearchTable
Dim SQL As String, rs As Recordset, db As DATABASE

If (LastGroupId = GroupID) Then
  ' Returned saved result if function is called with the
  ' same GroupID more than once in a row.
  NthInGroup = LastNthInGroup
Else
  ' Set the SQL statement parameters. These are the only items
  ' that need to be customized in this function.
  ' Set to Item field name.
  ItemName = "OrderDate"
  ' Set to Group ID field name.
  GroupIDName = "CustomerID"
  ' GroupID Delimiter Character:
  ' For Text use "'", Date "#", Numeric ""
  GDC = "'"
  ' Set to search table.
  SearchTable = "Orders"
  ' Build a Top N SQL statement dynamically given N and
  ' GroupID as parameters. Note that the sort is by the
  ' item in descending order, in order to get the Top N
  ' largest items.
  SQL = "Select Top " & N & " [" & ItemName & "] "
  SQL = SQL & "From [" & SearchTable & "] "
  SQL = SQL & "Where [" & GroupIDName & "]=" & GDC & GroupID & GDC & " "
  SQL = SQL & "Order By [" & ItemName & "] Desc"
  ' Open up recordset on Top N SQL statement and read the
  ' last record to get the smallest item in the Top N.
  Set db = CurrentDb()
  Set rs = db.OpenRecordset(SQL)
  If (rs.BOF And rs.EOF) Then
     ' No matches found, return a null.
     LastNthInGroup = Null
     LastGroupId = GroupID
     NthInGroup = LastNthInGroup
     Else
        ' Return the smallest Top N item in the group.
        rs.MoveLast
        LastNthInGroup = rs(ItemName)
        LastGroupId = GroupID
        NthInGroup = LastNthInGroup
      End If
   End If

End Function

Compile the module, and then close and save the module as basTopN.
Click Queries, and then click New.
Click Design View, and then click OK.
In the Show Table dialog box, click Customers and Orders, click Add, and then click Close.
Add the following fields to the query grid:
Field: CustomerID
Sort: Ascending

Field: OrderID

Field: Order Date
Sort: Descending
Criteria: >= NthInGroup([Customers].[CustomerID],5)
Run the query.

Note that for all customers who have at least five orders, the query returns the five most recent orders. For customers with fewer than five orders, the query returns all orders.

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REFERENCES

For more information about subqueries, click Microsoft Access Help on the Help menu, type sql subqueries in the Office Assistant or the Answer Wizard, and then click Search to view the topic.

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