Auto generate id number in vb.net - vb.net

Question

I am creating a library management system. I want to generate different book code for different book genre. For eg:- There is a combo box with book three book genres(novel,literature,poem) and there is another text box with book code. I want if someone chooses novel in comboBox, book code starts form N001 and if someone chooses literature in comboBox, book code starts from L001 and if someone chooses poem, book code starts from P001.
(My vb application is connected with ms access database.)

Answer 1

I see several problems with your fundamental numbering strategy.
Most library numbering systems (Dewey Decimal/Library of Congress) factor sort ordering abilities to their numbering system and allow for future additions to be able to sort higher without needing to shift the number of all other items. With your incremental strategy, how would you accommodate inserting a book later in the process?
If you set up with three digits of numbering, what would you do once you get 1000 books in a category?
While you could use a magic number table that includes the last id allocated per category and increment it as you add a new book. If you have more than one user, there is a potential for a race condition where two users get the same last number and both try to increment it at the same time landing on the same new Id.
I typically recommend keeping a generic ID that has no meaning in the system. This number would be set as an Auto Number in Access and the database would generate new Ids when records are added and ensure that you don't have the collision issue above. If you absolutely need a publically viewable Id, you can use the magic number strategy for a separate BookNumber column that isn't used for record identity. That way if you have a collision in the future, data won't get corrupted.

Answer 2

there are several schools of thought on how one can approach this.
One way? Well, you say have a book type or some such, and then you have that book code. It would be a simple number column without the prefix (P, B etc.).
So, to get the next number, you would then go:
myBookNumber = DMAX("BookNumber","tblBooks","BookType = 'P')
MyBookNumber = MyBookNumber + 1
So, you simply store the book number separate from the prefix. In forms, reports etc., you can then always display the book number like rst!BookType & rst!BookNumber
Another way, which I prefer is to create a table that allows you to get the next book number This also makes it easy to add more types, and better is such a design lets you set the starting number. This approach is also often used for invoice numbers.
So, you create a table called tlbBookIncrements. It will look like this:
ID: (pk - autonumber)
BookType: text (the one char book type)
BookNumber: Number the next book number to assign.
You then build a function like this:
Public Function GetNextBookN(strBookType As String) As Long
Dim strSQL As String
Dim rst As DAO.Recordset
strSQL = "SELECT * FROM tblBookIncrement where BookType = '" & strBookType & "'"
Set rst = CurrentDb.OpenRecordset(strSQL)
If rst.RecordCount = 0 Then
' this book type does not exist - lets add it and start at 1
rst.AddNew
rst!BookType = strBookType
rst!booknumber = 1
End If
GetNextBookN = rst!booknumber
rst!booknumber = rst!booknumber + 1
rst.Update
rst.Close
End Function
Update:
The poster was using vb.net, not VBA, my bad.
Here is the above as vb.net, it really much the same.
Public Function GetNextBookN(strBookType As String) As Integer
Using mycon As New OleDbConnection(My.Settings.ConnectionString1)
Dim strSQL As String
strSQL = "SELECT * FROM tblBookIncrement where BookType = '" & strBookType & "'"
Using da As New OleDbDataAdapter(strSQL, mycon)
Dim cmdBuilder = New OleDbCommandBuilder(da)
Dim rst As New DataTable
da.Fill(rst)
da.AcceptChangesDuringUpdate = True
With rst
If .Rows.Count = 0 Then
With .Rows.Add
.Item("BookType") = strBookType
.Item("booknumber") = 1
End With
GetNextBookN = 1
Else
GetNextBookN = .Rows(0).Item("booknumber")
.Rows(0).Item("booknumber") += 1
End If
End With
da.Update(rst)
End Using
End Using
End Function
So, now anytime you need a book number, you can go like this:
Say, in a form, you need to assign the book number, you could go like this:
You could say use the on-insert event of the form, and go like this:
me!BookNumber = me!bookType & GetNextBookN(me!bookType)
So, the above would say if the bookType is "P", result in P101 if the numbering table has a book type of P and the booknumber set to 101. And of course once you pull that number, then the handy dandy function will increment the number for you.
And I also added the feature that if the type requested does not exist, then we add the type and start at 1. This way, the code works as you over time add new types of books, and then the book number table will always work, and have available a next book number easily available in code.