Visual Basic 6 and dbf: issues with 'join' and 'where' - sql

Question

I need to modify an old vb6 application that needs to import some data from a dBase IV database.
In the past the selection query involved a single table (dbf file) and used to work perfectly.
Now i need to edit this query to introduce a join on a second table, using multiple fields.
This is a simplified version of my code:
Dim cnn As New Connection
Dim rs As New Recordset
Dim sql As String
cnn.Open "Provider=Microsoft.Jet.OLEDB.4.0;Extended properties=dBase IV;Data source=d:\100\db;"
sql = "..." 'see below!
rs.CursorLocation = adUseClient
rs.Open sql, cnn, adOpenDynamic, adLockOptimistic
debug.print rs.RecordCount
rs.Close
cnn.Close
These two tables have a typical master-detail structure; I checked the db documentation and inspected TABFAT01 and TABFAT02, so I can assume that:
Join between these tables is [TABFAT01] 1 <-> n [TABFAT02], and is done on
TIPDOC (text), ANNDOC (text) and NUMDOC (numeric) fields.
Each row in TABFAT01 has at least 1 joined row in TABFAT02.
Each row in TABFAT02 has 1 joined row in TABFAT02.
TABFAT01 has 63 records.
TABFAT02 has 907 records.
First issue
My first query is:
select t.TIPDOC, t.NUMDOC, t.ANNDOC, t.DATDOC, t.LIBER03, c.LIBER04
from TABFAT01 t inner join
TABFAT02 c on t.TIPDOC = c.TIPDOC and t.ANNDOC = c.ANNDOC and t.NUMDOC = c.NUMDOC
This query returns 0 records.
If I change conditions order this way:
select t.TIPDOC, t.NUMDOC, t.ANNDOC, t.DATDOC, t.LIBER03, c.LIBER04
from TABFAT01 t inner join
TABFAT02 c on t.ANNDOC = c.ANNDOC and t.NUMDOC = c.NUMDOC and t.TIPDOC = c.TIPDOC
the query returns 907 records.
I don't understand how and why conditions order has impact on query results.
Second issue
If I add a where clause:
select t.TIPDOC, t.NUMDOC, t.ANNDOC, t.DATDOC, t.LIBER03, c.LIBER04
from TABFAT01 t inner join
TABFAT02 c on t.ANNDOC = c.ANNDOC and t.NUMDOC = c.NUMDOC and t.TIPDOC = c.TIPDOC
where c.LIBER04 = 'a'
the query returns 0 records.
However, if I run this query:
select * from TABFAT02 c where LIBER04 = 'a'
it returns 1 record, with TIPDOC = 'F2', ANNDOC = '2018', NUMDOC = 1854.
A subsequent query:
select * from TABFAT01 t where t.TIPDOC = 'F2' and t.ANNDOC = '2018' and t.NUMDOC = 1854
returns 1 record, as expected.
This happens for every field I tried to put in where clause with joined tables, except TIPDOC.
If I filter by TIPDOC, results are correct.
Third issue
The first time I run my code after opening vb6 IDE it gives the following error:
Run-time error '-2147467259 (80004005)': Selected Collating sequence Not Supported by the operating system.
(I actually use the Italian version of vb6, and the original error message says "Sequenza di ordinamento selezionata non supportata dal sistema operativo.".
I guess that the message I written above is the right match for the English version.)
This error is not displayed on subsequent runs, until I close and reopen vb6.

Answer 1

I agree with thx1138v2 about not specifically using the Jet OleDB driver, but one more specific to the source. You mention dBASE IV. Is this accurate or is it really with Visual FoxPro. You could confirm if the table has indexes and note (memo) fields the file names would be suffixed as
YourTable.dbf (actual table)
YourTable.cdx (compound index file)
YourTable.fpt (notes/memo file content is table has such columns).
Having confirmed, and if it IS Visual Foxpro, I would get the Microsoft driver directly from Microsoft
If the data table is being updated from a source outside VFP, then it is possible that the indexes may be out of sync as the other drivers won't necessarily open the index file (cdx) correctly to keep the new records in synch. So when trying to do the join based on one series of join clauses it does use the non-synched index. But changing the order forces to default to the natural order, thus seeing all possible records to resolve.
See and hopefully this makes sense and helps to tune-in on the missing link in your querying issue.

Answer 2

First clear up your error message. "Selected Collating sequence Not Supported by the operating system." probably means you didn't specify a collating sequence or the Jet.OLEDB driver doesn't support whatever collating sequence you specified.
I never got the Jet.OLEDB drivers to work with my application. My guess is because the files I was dealing with were created by FoxPro. So I used the Visual Fox Pro ODBC drivers. Here's an example connection.
"DSN=BCLVariance;UID=;SourceDB=C:\AMSI\BCLTemp;SourceType=DBF;Exclusive=Yes;BackgroundFetch=No;Collate=Machine;Null=Yes;Deleted=Yes;"
You will see that it allows you to control various parameters that you aren't controlling in your Jet.OLEDB connection string. The ODBC data source is setup under System Data Sources tab in ODBC Data Source Administrator.
I never understood exactly why but the sequence you state the table names in a SQL query does make a difference. It has to do with how the internal query interpreter handles them. You need to specify the detail table first and the master is joined to the detail. Like this:
select t.TIPDOC, t.NUMDOC, t.ANNDOC, t.DATDOC, t.LIBER03, c.LIBER04
from TABFAT02 c inner join
TABFAT01 t on t.TIPDOC = c.TIPDOC and t.ANNDOC = c.ANNDOC and t.NUMDOC = c.NUMDOC
That may also clear up your other problems. Give it a try.
BTW, you'll also need to change your initial code to:
if Not rs.BOF then
rs.MoveLast
debug.print "rs.RecordCount=";rs.RecordCount
else
debug.print "rs.BOF = True"
end if