Merging records with consecutive dates - ms-access

Question

I have the following problem in a Database using Access 2007 as front end and SQL2005 as back-end.
In Table A I have the following data structure
Table A
ID Date Supplier_ID
1 10/22/2009 1
2 10/23/2009 1
3 10/24/2009 2
4 10/25/2009 2
5 10/26/2009 1
I need to merge values that have consecutive dates and the same Supplier_ID, increment a new column (Days) for each of these consecutive records and them write the data into Table B, so that I have
Table B
ID Date Supplier_ID Days
1 10/22/2009 1 2
2 10/24/2009 2 2
3 10/26/2009 1 1
Only consecutive days should be merged. Hence ID 5 in table A has is added to table B as a new record. It's been some time since I've been working with Access VBA and wondered what the right approach to this would be.

Answer 1

Here's some ANSI-92 Query Mode syntax SQL DDL and DML to recreate the table and a possible solution:
CREATE TABLE TableA
(
ID INTEGER NOT NULL UNIQUE,
[Date] DATETIME NOT NULL,
Supplier_ID INTEGER NOT NULL
);
INSERT INTO TableA (ID, [Date], Supplier_ID)
SELECT DT1.ID, DT1.[Date], DT1.Supplier_ID
FROM (
SELECT DISTINCT 1 AS ID, '2009-10-22 00:00:00' AS [Date], 1 AS Supplier_ID FROM Customers
UNION ALL
SELECT DISTINCT 2, '2009-10-23 00:00:00', 1 FROM Customers
UNION ALL
SELECT DISTINCT 3, '2009-10-24 00:00:00', 2 FROM Customers
UNION ALL
SELECT DISTINCT 4, '2009-10-25 00:00:00', 2 FROM Customers
UNION ALL
SELECT DISTINCT 5, '2009-10-26 00:00:00', 1 FROM Customers
) AS DT1;
CREATE VIEW TableA_StartDates (Supplier_ID, start_date)
AS
SELECT T1.Supplier_ID, T1.[Date]
FROM TableA AS T1
WHERE NOT EXISTS (
SELECT *
FROM TableA AS T2
WHERE T2.Supplier_ID = T1.Supplier_ID
AND DATEADD('D', -1, T1.[Date]) = T2.[Date]
);
CREATE VIEW TableA_EndDates (Supplier_ID, end_date)
AS
SELECT T3.Supplier_ID, T3.[Date]
FROM TableA AS T3
WHERE NOT EXISTS (
SELECT *
FROM TableA AS T4
WHERE T4.Supplier_ID = T3.Supplier_ID
AND DATEADD('D', 1, T3.[Date]) = T4.[Date]
);
CREATE VIEW TableA_Periods (Supplier_ID, start_date, end_date)
AS
SELECT DISTINCT T5.Supplier_ID,
(
SELECT MAX(S1.start_date)
FROM TableA_StartDates AS S1
WHERE S1.Supplier_ID = T5.Supplier_ID
AND S1.start_date <= T5.[Date]
),
(
SELECT MIN(E1.end_date)
FROM TableA_EndDates AS E1
WHERE E1.Supplier_ID = T5.Supplier_ID
AND T5.[Date] <= E1.end_date
)
FROM TableA AS T5;
SELECT P1.Supplier_ID, P1.start_date, P1.end_date,
DATEDIFF('D', P1.start_date, P1.end_date) + 1 AS Days
FROM TableA_Periods AS P1;

Answer 2

You have a lot of business rules with some unique assumptions. Eg TableA is never empty, TableB is always empty before you run this
Regardless, the code below will work using your sample data:
Dim rs As Recordset
Dim dbs As Database, qdf As QueryDef, strSQL As String
Set dbs = CurrentDb
Dim Supplier_ID As Integer
Dim Days As Integer
Dim FirstDate As Date
Set qdf = dbs.CreateQueryDef("", "select [Date], Supplier_ID from tableA order by [date] ")
Set rs = qdf.OpenRecordset()
Supplier_ID = rs!Supplier_ID
FirstDate = rs!Date
While Not rs.EOF
If rs!Supplier_ID <> Supplier_ID Then
If Supplier_ID <> 0 Then
' we don't want to insert the first time we run through this, so we check if Supplier_ID is not zero
dbs.Execute ("insert into tableB ([Date], Supplier_ID, Days) select #" + Format(FirstDate, "dd-mmm-yyyy") + "#, " + Str(Supplier_ID) + ", " + Str(Days))
Supplier_ID = rs!Supplier_ID
FirstDate = rs!Date
Days = 0
End If
End If
Days = Days + 1
rs.MoveNext
Wend
dbs.Execute ("insert into tableB ([Date], Supplier_ID, Days) select #" + Format(FirstDate, "dd-mmm-yyyy") + "#, " + Str(Supplier_ID) + ", " + Str(Days))
While you can do it, Access (and generally SQL) isn't the best for doing row by row comparisons. Even though you can use cursors or code like the example above, maintenance can be a challenge!

Answer 3

I don't think this can be done in a single SQL, so you'll have to write some code. My first idea would be to make some kind of dictionary (Key = SupplierID, Value = (FirstDate, LastDate)) and use an algorithm that iterates through the data in Table A and fills the dictionary, kind of like this (pseudo-code):
records = recordset: SELECT * FROM Table A ORDER BY Date
Do Until records.EOF
If dictionary contains records!SupplierID
If dictionaryEntry.LastDate = records!Date - 1
dictionaryEntry.LastDate = records!Date
Else
Make Table B Entry (Days = LastDate - FirstDate) and remove dictionary entry
End If
Else
Create dictionary entry for this Supplier with LastDate = FirstDate = records!Date
End If
records.MoveNext
Loop
records.Close
Make Table B Entries for all remaining entries in the dictionary
For the dictionary, you could use a Scripting.Dictionary with the value being a 2-Element-Array.