Identify the maximum number in a field before a gap in an sequence on before insert event using vba

I tried all day searching forums to find an answer but  I couldn't find any.. I hope someone could help me here.
the "BookID" is a field in the table for auto generated ID number for the book  This is what I' m using at the moment

Private Sub Form_BeforeInsert(Cancel As Integer)
TxBook = Nz(DMax("BookID", "Tbl_Books"), 0) + 1  'and works fine
end sub


Works fine but what I need is  if one book or more is deleted I want it to inset the lowest missing number then if nothing misiing will inset the maximum number
Ronald MalkAsked:
Who is Participating?
 
hnasrConnect With a Mentor Commented:
@Ronald Malk
Try this sample database. If still in doubt attach a representative copy of your database.
Check the code in the before insert event.
Private Function nextValue()
    'this code selects the minimum value of a filed in a table
    'where the next value is missing from the table
    'x and y are called aliases to allow using the same table in different nested queries
    'so the x.fld is compared to the y.fld
    Dim rs2 As Recordset
    Set rs2 = CurrentDb.OpenRecordset("SELECT Min(x.BookID)+1 FROM Tbl_Books AS x WHERE (((Exists (Select BookID from Tbl_Books y WHERE y.BookID=x.BookID+1))=False))")
    nextValue = rs2(0)
End Function
Private Sub Form_BeforeInsert(Cancel As Integer)
    BookID = nextValue()
End Sub

Open in new window

TblBooks.accdb
1
 
Ryan ChongCommented:
>>Works fine but what I need is  if one book or more is deleted I want it to inset the lowest missing number then if nothing misiing will inset the maximum number

so you mean to reuse the so called "deleted id" for your new entry? from my opinion, you should use the new id instead of re-using the "deleted id" to maintain your data integrity.
1
 
Gustav BrockCIOCommented:
Yes, seems like a very bad idea.

Leave the old Id, but mark it as Deleted or Obsolete (a boolean field).
Then proceed as you do now.

For a current list of books, filter on Obsolete = False

/gustav
1
Free Tool: IP Lookup

Get more info about an IP address or domain name, such as organization, abuse contacts and geolocation.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

 
Jim Dettman (Microsoft MVP/ EE MVE)President / OwnerCommented:
An auto number for a primary key is a meaningless key and you're trying to apply meaning to it by having the sequence represent something. That's why you're not finding any solutions on this.

 There's no reason to do what you're trying to do,  and as Ryan pointed out good reasons for not doing it.

As Gustav  pointed out, there are other ways to handle situations that you might have. What is it that you're trying to achieve by reusing these old numbers?

Jim
0
 
Gustav BrockCIOCommented:
It should be noted that - if you insist - it can easily be done.
You can create a list of missing numbers:
Private Sub btnSearch_Click()

' Read table/query sequentially to 
' record all missing IDs.
' Fill a ListBox with missing values.
' A reference to Microsoft DAO must be 
' present.

  ' Define search table or query.
  Const cstrTable As String = "Orders"
  Const cstrField As String = "OrderID"
  
  Dim dbs As DAO.Database
  Dim rst As DAO.Recordset
  Dim lst As ListBox
  Dim col As Collection
  
  Dim strSQL As String
  Dim strList As String
  Dim lngLast As Long
  Dim lngNext As Long
  Dim lngMiss As Long
    
  strSQL = "Select " & cstrField & "" _
   & " From " & cstrTable & " _
   & " Order By 1;"  
  
  Set lst = Me!lstMissing
  Set col = New Collection
  Set dbs = CurrentDb
  Set rst = dbs.OpenRecordset(strSQL)
  
  If rst.RecordCount = 0 Then
    'The recordset is empty.
    'Nothing to do.
  Else
    lngLast = rst(cstrField).Value
    rst.MoveNext
    While rst.EOF = False
      lngNext = rst(cstrField).Value
      For lngMiss = lngLast + 1 To _
       lngNext - 1
        col.Add (lngMiss)
      Next
      lngLast = lngNext
      rst.MoveNext
    Wend
      'Generate next value in sequence.
      'Discard if collecting only 
      'missing values.
      col.Add (lngLast + 1)
  End If
  rst.Close
  
  'Populate list box from collection.
  For lngMiss = 1 To col.Count
    If Len(strList) > 0 Then 
      strList = strList & ";"
    End If
    strList = strList & col(lngMiss)
    Debug.Print col(lngMiss)
  Next
  lst.RowSource = strList
  Debug.Print strList
  
  Set rst = Nothing
  Set dbs = Nothing
  Set col = Nothing
  Set lst = Nothing
  
End Sub

Open in new window

The full story is here:

Find and Generate Missing Values in an Access Table

/gustav
0
 
hnasrCommented:
For a table a(..., f3)
f3
1
2
4
5

This query gives the next lowest missing number:
SELECT Min(x.f3)+1 AS Expr1
FROM a AS x
WHERE (((Exists (Select f3 from a y WHERE y.f3=x.f3+1))=False));

Open in new window

Result 3
This a VBA code for a function to return the next value as a variable:

Private Function nextValue()
    Dim rs2 As Recordset
    Set rs2 = CurrentDb.OpenRecordset("SELECT Min(x.f3)+1 FROM a AS x WHERE (((Exists (Select f3 from a y WHERE y.f3=x.f3+1))=False))")
    nextValue = rs2(0)
End Function

'to use, you may set txtBox1 = nextValue()
Private Sub Command40_Click()
     Debug.Print nextValue()
End Sub

Open in new window

Result 3
0
 
Ronald MalkAuthor Commented:
Thank you so much it works very good, It's something new for me, First time I do VBA this way, I still need to teach my self more of the internet that where I'm learning from.
0
 
hnasrCommented:
Welcome!
0
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

All Courses

From novice to tech pro — start learning today.