Question

I have a query to return random distinct rows from an Access database. Here is the query:

SELECT * FROM  (SELECT DISTINCT m.MemberID, m.Title, m.FullName, m.Address,          m.Phone, m.EmailAddress, m.WebsiteAddress FROM Members AS m INNER JOIN MembersForType AS t ON m.MemberID = t.MemberID WHERE  (Category = 'MemberType1' OR Category = 'MemberType2')) as Members  ORDER BY RND(members.MemberID) DESC 

When I run this in Access it returns the rows in different order every time, as per the random sort order. When I run it through my web app however the rows return in the same order every time. Here is how I call it in my code-behind:

private void BindData() {     using (AccessDataSource ds = new AccessDataSource('~/App_Data/mydb.mdb', GetSQLStatement()))     {         ds.DataSourceMode = SqlDataSourceMode.DataReader;         ds.CacheDuration = 0;         ds.CacheExpirationPolicy = DataSourceCacheExpiry.Absolute;         ds.EnableCaching = false;                     listing.DataSource = ds.Select(new DataSourceSelectArguments());         listing.DataBind();         if (listing.Items.Count == 0)             noResults.Visible = true;         else             noResults.Visible = false;     } } 

I added in all that stuff about caching because I thought maybe the query was being cached but the result was the same. I put a breakpoint in the code to make sure the query was the same as above and it was.

Any ideas? This is driving me nuts.

Answer 1

When executing the ACE/Jet RND function against a new connection the same seed value is used each time. When using MS Access you are using the same connection each time, which explains why you get a different value each time.

Consider these VBA examples: the first uses a new connection on each iteration:

Sub TestDiff()    Dim con As Object   Set con = CreateObject('ADODB.Connection')   With con     .ConnectionString = _         'Provider=MSDataShape;Data ' & _         'Provider=Microsoft.ACE.OLEDB.12.0;' & _         'Data Source=C:\Tempo\Test_Access2007.accdb'     .CursorLocation = 3      Dim i As Long     For i = 0 To 2        .Open       Debug.Print .Execute('SELECT RND FROM OneRowTable;')(0)       .Close     Next   End With  End Sub 

Output:

 0.705547511577606   0.705547511577606   0.705547511577606  

Note the same value each time.

The second example uses the same connection on each iteration (the .Open and .Close statements are relocated outside the loop):

Sub TestSame()    Dim con As Object   Set con = CreateObject('ADODB.Connection')   With con     .ConnectionString = _         'Provider=MSDataShape;Data ' & _         'Provider=Microsoft.ACE.OLEDB.12.0;' & _         'Data Source=C:\Tempo\Test_Access2007.accdb'     .CursorLocation = 3      .Open      Dim i As Long     For i = 0 To 2       Debug.Print .Execute('SELECT RND FROM OneRowTable;')(0)     Next      .Close    End With  End Sub 

Output:

 0.705547511577606   0.533424019813538   0.579518616199493  

Note different values each time.

In VBA code you can use the Randomize keyword to seed the Rnd() function but I don’t think this can be done in ACE/Jet. One workaround is to use the least significant decimal portion of the ACE/Jet the NOW() niladic function e.g. something like:

SELECT CDBL(NOW()) - ROUND(CDBL(NOW()), 4) FROM OneRowTable