Microsoft Access: How to create new table that has 1 of every 5 records from the original? - ms-access

Question

I have a large table and want to create a smaller table that systematically picks out one of every 5 records from the original. How best to do this?
This means it should have every row with id ending in 0 and 5 for example.
Thanks.

Answer 1

Assuming that your IDs as auto-incremented and continuous (with no deleted records), the simplest way could something like:
SELECT *
FROM BigTable
WHERE ID Mod 5 = 0
You would insert that into the small table like this:
INSERT INTO SmallTable
SELECT *
FROM BigTable
WHERE ID Mod 5 = 0
If your primary key ID is not sequential you can do this:
SELECT BigTable.*
FROM (SELECT ID,
(SELECT COUNT(ID) + 1
FROM BigTable AS A
WHERE A.ID < B.ID) AS RowNum
FROM BigTable AS B) AS C
INNER JOIN BigTable
ON C.ID = BigTable.ID
WHERE RowNum Mod 5 = 0
Note though that this is OK for a one-time thing as it is quite slow if you have a lot of records.
Better do it in code if you have lots of records (or maybe you have no primary key, which is usually bad btw).
The code below is generic code to copy all records from one table into another (existing) table:
Public Sub CreateSmallTable(largeTableName As String, _
smallTableName As String, _
interval As Integer)
Dim rsL As DAO.RecordSet
Dim rsS As DAO.RecordSet
Dim db As DAO.Database
Dim rowNum As Integer
Set db = CurrentDb()
' Open the LargeTable in read-only mode '
Set rsL = db.OpenRecordset(largeTableName, dbOpenSnapshot)
If Not (rsL Is Nothing) Then
Set rsS = db.OpenRecordset(smallTableName, dbOpenTable, dbAppendOnly)
With rsL
' We'll use a generic way to copy all fields from the BigTable '
Dim fd As DAO.field
Dim flds As DAO.Fields
Set flds = rsL.Fields
.MoveFirst
' For each record in the BigTable '
Do While Not .EOF()
If rowNum Mod interval = 0 Then
rsS.AddNew
' Copy all fields from BigTable to SmallTable '
For Each fd In flds
rsS.Fields(fd.Name) = .Fields(fd.Name)
Next fd
rsS.Update
End If
rowNum = rowNum + 1
.MoveNext
Loop
.Close
End With
rsS.Close
Set rsS = Nothing
Set rsL = Nothing
End If
End Sub
And call it like that (assuming that both BigTable and SmallTable have the same structure and that there are no existing records in SmallTable that could create a primary key violation when trying to inster duplicate records):
CreateSmallTable "Bigtable", "Smalltable", 5
Edit: following HansUp response, changed SQL queries to use Mod instead of its functional version: (Round(ID/5) = ID/5). He's right, no need to make it more complex than it needs to be.

Answer 2

Consider id Mod 5, and select only those where the remainder is zero.
SELECT f.id, f.foo_text
FROM tblFoo AS f
WHERE (id Mod 5) = 0;