SQL server multi-period comparison - sql-server

Question

I have the following table T1 (sample shown), which shows the category for each client (each with a unique ID) on a specific date and his category on the next date:
+------------+----------------+----------+---------------+
| DATE | ID | STAGE | STAGE_NEXT |
+------------+----------------+----------+---------------+
| 2014-07-01 | 10010101841033 | 1 | 1 |
| 2015-07-01 | 74610108542146 | 1 | 1 |
| 2014-10-01 | 47970108841775 | 3 | 3 |
| 2014-10-01 | 48870108841816 | 2 | 3 |
| 2014-10-01 | 32910097439541 | 1 | 1 |
| 2016-04-01 | 46930097440855 | 2 | 3 |
| 2016-04-01 | 47380097440931 | 2 | 3 |
| 2016-04-01 | 54560097441411 | 3 | 3 |
+------------+----------------+----------+---------------+
Table info:
- Rows: 513,000
- Date range: 2013-01-01 to 2019-10-01
- Stages: 1 - 3
I need to create a new column in T1, which will flag the date a client moved to Stage 1 if at any point he was in Stage 3. For example if we take 1 client from T1 by using this code:
SELECT [DATE], ID, STAGE, STAGE_NEXT
FROM T1
WHERE ID = '74610108542146'
ORDER BY [DATE]
We get the following result:
+------------+----------------+-------+------------+
| DATE | ID | STAGE | STAGE_NEXT |
+------------+----------------+-------+------------+
| 2015-07-01 | 74610108542146 | 1 | 1 |
| 2015-10-01 | 74610108542146 | 1 | 1 |
| 2016-01-01 | 74610108542146 | 1 | 2 |
| 2016-04-01 | 74610108542146 | 2 | 1 |
| 2016-07-01 | 74610108542146 | 1 | 1 |
| 2016-10-01 | 74610108542146 | 1 | 2 |
| 2017-01-01 | 74610108542146 | 2 | 3 |
| 2017-04-01 | 74610108542146 | 3 | 3 |
| 2017-07-01 | 74610108542146 | 3 | 2 |
| 2017-10-01 | 74610108542146 | 2 | 1 |
| 2018-01-01 | 74610108542146 | 1 | 1 |
| 2018-04-01 | 74610108542146 | 1 | NULL |
+------------+----------------+-------+------------+
After the new column with the flag is added to T1 we should be able to get the following result using this code on T1:
SELECT [DATE], ID, STAGE, STAGE_NEXT, FLAG
FROM T1
WHERE ID = '74610108542146'
ORDER BY [DATE]
+------------+----------------+-------+------------+------+
| DATE | ID | STAGE | STAGE_NEXT | FLAG |
+------------+----------------+-------+------------+------+
| 2015-07-01 | 74610108542146 | 1 | 1 | 0 |
| 2015-10-01 | 74610108542146 | 1 | 1 | 0 |
| 2016-01-01 | 74610108542146 | 1 | 2 | 0 |
| 2016-04-01 | 74610108542146 | 2 | 1 | 0 |
| 2016-07-01 | 74610108542146 | 1 | 1 | 0 |
| 2016-10-01 | 74610108542146 | 1 | 2 | 0 |
| 2017-01-01 | 74610108542146 | 2 | 3 | 0 |
| 2017-04-01 | 74610108542146 | 3 | 3 | 0 |
| 2017-07-01 | 74610108542146 | 3 | 2 | 0 |
| 2017-10-01 | 74610108542146 | 2 | 1 | 1 |
| 2018-01-01 | 74610108542146 | 1 | 1 | 0 |
| 2018-04-01 | 74610108542146 | 1 | NULL | 0 |
+------------+----------------+-------+------------+------+
If the client never moved to Stage 3 then the flag for the client is always 0

Answer 1

You could calculate and update the new FLAG column from a CTE.
The update statement uses the LAG function to use the previous STAGE in the calculation of FLAG.
;WITH CTE AS
(
SELECT ID, [DATE], FLAG,
CASE
WHEN STAGE = 2
AND STAGE_NEXT = 1
AND LAG(STAGE) OVER (PARTITION BY ID ORDER BY IIF(STAGE=2 AND STAGE_NEXT=2,0,1), [DATE]) = 3
THEN 1
ELSE 0
END AS CalcFlag
FROM T1
WHERE ID = '10010101841033' -- optional, to target only 1 ID
)
UPDATE CTE
SET FLAG = CalcFlag
WHERE (FLAG IS NULL OR FLAG != CalcFlag);
The IIF(STAGE=2 AND STAGE_NEXT=2,0,1) in the LAG is used to make the calculation also work when the stage 2 is repeated.
Test it on rextester here

Answer 2

Try this,
DECLARE #T1 table
(
[DATE] date,ID numeric(18,0),STAGE int,STAGE_NEXT int
)
INSERT INTO #T1 VALUES
('2013-01-01',10010101841033,1,1 ),
('2013-04-01',10010101841033,1,3 ),
('2013-07-01',10010101841033,3,3 ),
('2013-10-01',10010101841033,3,2 ),
('2014-01-01',10010101841033,2,1 ),
('2014-04-01',10010101841033,1,1 ),
('2014-07-01',10010101841033,1,1 ),
('2014-10-01',10010101841033,1,NULL),
('2014-07-01',47820108841771,1,2)
SELECT A.DATE,A.ID,A.STAGE,A.STAGE_NEXT,
CASE WHEN B.ID IS NOT NULL AND (STAGE_NEXT=1 AND STAGE>STAGE_NEXT) THEN 1 ELSE 0 END AS FLAG
FROM #T1 A
LEFT JOIN
(
SELECT DISTINCT ID AS ID
FROM #T1
WHERE STAGE_NEXT=3
)B
ON A.ID=B.ID