How to select rows and nearby rows with specific conditions - sql

Question

I have a table (Trans) of values like
OrderID (unique) | CustID | OrderDate| TimeSinceLast|
------------------------------------------------------
123a | A01 | 20.06.18 | 20 |
123y | B05 | 20.06.18 | 31 |
113k | A01 | 18.05.18 | NULL | <------- need this
168x | C01 | 17.04.18 | 8 |
999y | B05 | 15.04.18 | NULL | <------- need this
188k | A01 | 15.04.18 | 123 |
678a | B05 | 16.03.18 | 45 |
What I need is to select the rows where TimeSinceLast is null, as well as a row preceding and following where TimeSinceLast is not null, grouped by custID
I'd need my final table to look like:
OrderID (unique) | CustID | OrderDate| TimeSinceLast|
------------------------------------------------------
123a | A01 | 20.06.18 | 20 |
113k | A01 | 18.05.18 | NULL |
188k | A01 | 15.04.18 | 123 |
123y | B05 | 20.06.18 | 31 |
999y | B05 | 15.04.18 | NULL |
678a | B05 | 16.03.18 | 45 |
The main problem is that TimeSinceLast is not reliable and for whatsoever reason does not calculate well the days since last order, so I cannot use it in a query for preceding or following row.
I have tried to look for codes and found something like this on this forum
with dt as
(select distinct custID, OrderID,
max (case when timeSinceLast is null then OrderID end)
over(partition by custID order by OrderDate
rows between 1 preceding and 1 following) as NullID
from Trans)
select *
from dt
where request_id between NullID -1 and NullID+1
But does not work well for my purposes. Also it looks like max function cannot work with missing values.
Many thanks

Answer 1

Use lead() and lag().
What I need is to select the rows where TimeSinceLast is null, as well as a row preceding and following where TimeSinceLast is not null.
First, the ordering is a little unclear. Your sample data and code do not match. The following assumes some combination of the date and orderid, but there may be other columns that better capture what you mean by "preceding" and "following".
This is a little tricky, because you don't want to always include the first and last rows -- unless necessary. So, look at two columns:
select t.*
from (select t.*,
lead(TimeSinceLast) over (partition by custid order by orderdate, orderid) as next_tsl,
lag(TimeSinceLast) over (partition by custid order by orderdate, orderid) as prev_tsl,
lead(orderid) over (partition by custid order by orderdate, orderid) as next_orderid,
lag(orderid) over (partition by custid order by orderdate, orderid) as prev_orderid
from t
) t
where TimeSinceLast is not null or
(next_tsl is null and next_orderid is not null) or
(prev_tsl is null and prev_orderid is not null);

Answer 2

USE APPLY
DECLARE #TransTable TABLE (OrderID char(4), CustID char(3), OrderDate date, TimeSinceLast int)
INSERT #TransTable VALUES
('123a', 'A01', '06.20.2018', 20),
('123y', 'B05', '06.20.2018' ,31),
('113k', 'A01', '05.18.2018' ,NULL), ------- need this
('168x', 'C01', '04.17.2018' ,8),
('999y', 'B05', '04.15.2018' ,NULL), ------- need this
('188k', 'A01', '04.15.2018' ,123),
('678a', 'B05', '03.16.2018' ,45)
SELECT B.OrderID, B.CustID, B.OrderDate, B.TimeSinceLast
FROM #TransTable A
CROSS APPLY (
SELECT 0 AS rn, A.OrderID, A.CustID, A.OrderDate, A.TimeSinceLast
UNION ALL
SELECT TOP 2 ROW_NUMBER() OVER (PARTITION BY CASE WHEN T.OrderDate > A.OrderDate THEN 1 ELSE 0 END ORDER BY ABS(DATEDIFF(day, T.OrderDate, A.OrderDate))) rn,
T.OrderID, T.CustID, T.OrderDate, T.TimeSinceLast
FROM #TransTable T
WHERE T.CustID = A.CustID AND T.OrderID <> A.OrderID
ORDER BY rn
) B
WHERE A.TimeSinceLast IS NULL
ORDER BY B.CustID, B.OrderDate DESC