Group-by data based on conditions - sql-server-2008

Question

I will be getting some data which I need to filter out based on some conditions. Sample data:
Cust_ID Date Result
1 2013-08-15 On hold
2 2013-08-16 NULL
3 2013-08-18 WIP
1 2013-08-20 Completed
3 2013-08-25 NULL
4 2013-08-28 NULL
4 2013-08-29 NULL
Conditions:
Fetch the distinct Cust_ID, based on latest Date.(i.e Max(Date))
If the Result is Null for latest Date then get the latest record with any other Result apart from NULL.
If the Result is NULL for all the records with same Cust_ID, pick the latest one based on Date
The desired output should be:
Cust_ID Date Result
1 2013-08-20 Completed
2 2013-08-16 NULL
3 2013-08-18 WIP
4 2013-08-29 NULL
Please advise.

Answer 1

You can do it easy with a CTE, note the CTE is not "needed" (you could use sub-queries) but I think it makes it clear what you are doing.
WITH NonNull AS
(
SELECT CustID, MAX(Date) as Date
FROM tablename
GROUP BY CustID
WHERE Result is not null
), Others AS
(
SELECT CustID, MAX(Date) as Date
FROM tablename
GROUP BY CustID
WHERE CustID NOT IN (SELECT CustID FROM NonNull)
), AlltogetherNow -- not really needed but clearer
(
SELECT CustID, Date
FROM NonNull
UNION ALL
SELECT CustID, Date
FROM Others
)
SELECT A.CustID, A.Date, J.Results
FROM AlltogetherNow A
JOIN tablename J ON A.CustID = J.CustID AND A.Date = J.Date

Answer 2

First you need an IS NULL indicator on each row:
SQL Fiddle
MS SQL Server 2008 Schema Setup:
CREATE TABLE dbo.Results
([CustID] int, [Date] datetime, [Result] varchar(9))
GO
INSERT INTO dbo.Results
([CustID], [Date], [Result])
VALUES
(1, '2013-08-15 00:00:00', 'On Hold'),
(2, '2013-08-16 00:00:00', NULL),
(3, '2013-08-18 00:00:00', 'WIP'),
(1, '2013-08-20 00:00:00', 'Completed'),
(3, '2013-08-25 00:00:00', NULL),
(4, '2013-08-28 00:00:00', NULL),
(4, '2013-08-29 00:00:00', NULL)
GO
Query 1:
SELECT *,CASE WHEN Result IS NULL THEN 0 ELSE 1 END IsNotNull
FROM dbo.Results
Results:
| CUSTID | DATE | RESULT | ISNOTNULL |
|--------|-------------------------------|-----------|-----------|
| 1 | August, 15 2013 00:00:00+0000 | On Hold | 1 |
| 2 | August, 16 2013 00:00:00+0000 | (null) | 0 |
| 3 | August, 18 2013 00:00:00+0000 | WIP | 1 |
| 1 | August, 20 2013 00:00:00+0000 | Completed | 1 |
| 3 | August, 25 2013 00:00:00+0000 | (null) | 0 |
| 4 | August, 28 2013 00:00:00+0000 | (null) | 0 |
| 4 | August, 29 2013 00:00:00+0000 | (null) | 0 |
Then you need to determine the first NULL row and the first NOT NULL row for each customer. You can use the ROW_NUMBER() function for that. You also need to know for each customer if there are any NOT NULL rows:
Query 2:
SELECT *,
ROW_NUMBER()OVER(PARTITION BY CustID,IsNotNull ORDER BY [Date] DESC) _rn,
COUNT(Result)OVER(PARTITION BY CustID) NotNullCount
FROM(
SELECT *,CASE WHEN Result IS NULL THEN 0 ELSE 1 END IsNotNull
FROM dbo.Results
)X1
Results:
| CUSTID | DATE | RESULT | ISNOTNULL | _RN | NOTNULLCOUNT |
|--------|-------------------------------|-----------|-----------|-----|--------------|
| 1 | August, 20 2013 00:00:00+0000 | Completed | 1 | 1 | 2 |
| 1 | August, 15 2013 00:00:00+0000 | On Hold | 1 | 2 | 2 |
| 2 | August, 16 2013 00:00:00+0000 | (null) | 0 | 1 | 0 |
| 3 | August, 25 2013 00:00:00+0000 | (null) | 0 | 1 | 1 |
| 3 | August, 18 2013 00:00:00+0000 | WIP | 1 | 1 | 1 |
| 4 | August, 29 2013 00:00:00+0000 | (null) | 0 | 1 | 0 |
| 4 | August, 28 2013 00:00:00+0000 | (null) | 0 | 2 | 0 |
Finally you can just filter out the first NOT NULL row if there are any or the first NULL row if there are no NOT NULL rows by using the calculated row number:
Query 3:
SELECT CustID,[Date],Result
FROM(
SELECT *,
ROW_NUMBER()OVER(PARTITION BY CustID,IsNotNull ORDER BY [Date] DESC) _rn,
COUNT(Result)OVER(PARTITION BY CustID) NotNullCount
FROM(
SELECT *,CASE WHEN Result IS NULL THEN 0 ELSE 1 END IsNotNull
FROM dbo.Results
)X1
)X2
WHERE _rn = 1 AND SIGN(NotNullCount) = IsNotNull
Results:
| CUSTID | DATE | RESULT |
|--------|-------------------------------|-----------|
| 1 | August, 20 2013 00:00:00+0000 | Completed |
| 2 | August, 16 2013 00:00:00+0000 | (null) |
| 3 | August, 18 2013 00:00:00+0000 | WIP |
| 4 | August, 29 2013 00:00:00+0000 | (null) |