SQL Server, assign key by date range - sql-server-2014

Question

BILLS:
billdate billno
-------------------
2015-04-13 8090
2015-03-11 8089
2015-02-14 8088
2015-02-10 8087
TRANSACTIONS
transactiondate billno
-----------------------
2015-04-15 8090
2015-04-13 8090
2015-04-12 8090
2015-04-10 8090
2015-03-12 8090
2015-03-11 8089
2015-03-10 8089
2015-02-11 8088
2015-02-01 8087
I need to assign billno to transaction line items as described above. Transactions outside the bill range -- i.e. those beyond 2015-04-13 that don't match up within the date range window need to go with the last available bill. And vice-versa, transactions on the bottom end such as 2015-02-01 need to go with the last available bill.
Not quite sure how to go about this query.

Answer 1

You can use either a cursor or a recursive common table expression (CTE). I'd advise saying away from cursors, so here's an example of the CTE.
There's a lot going on in this query so I'll try to break it down in pieces.
If you perform a FULL JOIN against the two tables you'll come up with something that starts to look like the answer, but there are a lot of NULL values
for each transaction:
DECLARE #Bills TABLE (billdate DATE, billno INT)
DECLARE #Transactions TABLE (transactiondate DATE)
INSERT INTO #Bills (billdate, billno) VALUES ('2015-04-13', 8090)
INSERT INTO #Bills (billdate, billno) VALUES ('2015-03-11', 8089)
INSERT INTO #Bills (billdate, billno) VALUES ('2015-02-14', 8088)
INSERT INTO #Bills (billdate, billno) VALUES ('2015-02-10', 8087)
INSERT INTO #Transactions (transactiondate) VALUES ('2015-04-15')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-04-13')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-04-12')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-04-10')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-03-12')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-03-11')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-03-10')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-02-11')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-02-01')
SELECT
b.billdate,
b.billno,
t.transactiondate,
COALESCE(b.billdate, t.transactiondate) AS [eventdate]
FROM #Bills b
FULL JOIN #Transactions t
ON b.billdate = t.transactiondate
ORDER BY eventdate DESC
To arrive at the solution we need to pull the previous event's billno to the current event, if the current event's billno IS NULL.
To accomplish this we can use the recursive CTE. Recursive CTEs don't allow joins within (http://msdn.microsoft.com/en-us/library/ms175972.aspx) them so I had to refactor the FULL JOIN to a UNION ALL, aliased as the "AnchorSet". The
CTE is INNER JOINed against itself, with an offset to the rownumber/eventdate, while the billno IS NULL.
DECLARE #Bills TABLE (billdate DATE, billno INT)
DECLARE #Transactions TABLE (transactiondate DATE)
INSERT INTO #Bills (billdate, billno) VALUES ('2015-04-13', 8090)
INSERT INTO #Bills (billdate, billno) VALUES ('2015-03-11', 8089)
INSERT INTO #Bills (billdate, billno) VALUES ('2015-02-14', 8088)
INSERT INTO #Bills (billdate, billno) VALUES ('2015-02-10', 8087)
INSERT INTO #Transactions (transactiondate) VALUES ('2015-04-15')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-04-13')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-04-12')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-04-10')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-03-12')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-03-11')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-03-10')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-02-11')
INSERT INTO #Transactions (transactiondate) VALUES ('2015-02-01')
SELECT
b.billdate,
b.billno,
t.transactiondate,
COALESCE(b.billdate, t.transactiondate) AS [eventdate]
FROM #Bills b
FULL JOIN #Transactions t
ON b.billdate = t.transactiondate
ORDER BY eventdate DESC
;WITH AnchorSet AS
(
SELECT
MIN(billdate) AS billdate,
MIN(billno) AS billno,
MIN(transactiondate) as transactiondate,
eventdate
FROM
(
SELECT
billdate,
billno,
transactiondate,
COALESCE(billdate, transactiondate) AS [eventdate]
FROM
(
SELECT
billdate,
billno,
NULL AS [transactiondate]
FROM #Bills b
UNION ALL
SELECT
NULL AS billdate,
NULL AS billno,
transactiondate
FROM #transactions t
)innerset
)innerset2
GROUP BY eventdate
)
, cte (billdate, billno, transactiondate, eventdate, rownumber, recursionlevel)
AS
(
SELECT
billdate,
billno,
transactiondate,
eventdate,
ROW_NUMBER() OVER (ORDER BY eventdate DESC) AS [rownumber],
0 AS [recursionlevel]
FROM
AnchorSet
WHERE
billno IS NULL
UNION ALL
SELECT
COALESCE(RecursiveSet.billdate, cte.billdate) AS [billdate],
COALESCE(RecursiveSet.billno, cte.billno) AS [billno],
COALESCE(RecursiveSet.transactiondate, cte.transactiondate) AS [transactiondate],
COALESCE(RecursiveSet.eventdate, cte.eventdate) AS [eventdate],
COALESCE(RecursiveSet.rownumber, cte.rownumber) AS [rownumber],
recursionlevel + 1 AS [recursionlevel]
FROM
(
SELECT
billdate,
billno,
transactiondate,
eventdate,
ROW_NUMBER() OVER (ORDER BY eventdate DESC) AS [rownumber]
FROM AnchorSet
)RecursiveSet
INNER JOIN cte ON cte.rownumber = RecursiveSet.rownumber - 1
)
SELECT
billno,
transactiondate
FROM
(
SELECT
billno,
transactiondate,
ROW_NUMBER() OVER (PARTITION BY transactiondate ORDER BY transactiondate DESC, rownumber DESC, recursionlevel DESC, billno DESC) AS [finalrownumber]
FROM cte
)SomeSet
WHERE finalrownumber = 1
ORDER BY transactiondate DESC
This will get you what you're looking for, with the exception that the final transaction will be null since it's after the last bill. You can create a query to get the latest billno and apply it to the remaining NULL values for the transaction's billnos. I'll leave that as an exercise for the reader.