Number of days since last activity - SQL - sql

Question

I'm trying to count the number of days since the last activity. My data is weekly aggregated. I'm able to do the lag but not able to include the current week.
Data:
ID DATE CHANNEL VENDOR ENG
xyz 2022-11-18 EMAIL ALPHA 1
xyz 2022-11-25 EMAIL ALPHA 1
xyz 2022-12-09 EMAIL ALPHA 1
xyz 2022-12-16 EMAIL ALPHA 0
xyz 2022-12-23 EMAIL ALPHA 0
xyz 2022-12-30 EMAIL ALPHA 3
I would like to have the output to be as follows:
ID DATE CHANNEL VENDOR ENG n_days
xyz 2022-11-18 EMAIL ALPHA 1 0
xyz 2022-11-25 EMAIL ALPHA 1 0
xyz 2022-12-09 EMAIL ALPHA 1 0
xyz 2022-12-16 EMAIL ALPHA 0 7
xyz 2022-12-23 EMAIL ALPHA 0 14
xyz 2022-12-30 EMAIL ALPHA 3 0
I have written a query but it not able to include the most latest week. Below is my query:
SELECT DISTINCT ID, DATE, CHANNEL, VENDOR,
DATE - LAG(DATE) OVER (PARTITION BY ID, CHANNEL, VENDOR ORDER BY DATE) AS "NDAYS_LAST_ENGAGED_CHANNEL_VENDOR"
FROM
tab1
WHERE
ENG>0

Answer 1

Your WHERE clause means it will only output the rows where eng > 0, so there is no way it will return what you want - its going to throw the last two rows away. And LAG will be just refer to the previous row in the partition, its not going to search for a row that matches your where clause as I think you hoped.
This should hopefully put you on the right track. I've assumed Sql server, though :
DROP TABLE IF EXISTS #t
CREATE TABLE #t (ID VARCHAR(3), [DATE] DATE, CHANNEL VARCHAR(5), VENDOR VARCHAR(5), ENG INT);
INSERT INTO #t VALUES
('xyz','20221118','EMAIL','ALPHA',1),
('xyz','20221125','EMAIL','ALPHA',0),
('xyz','20221209','EMAIL','ALPHA',1),
('xyz','20221216','EMAIL','ALPHA',0),
('xyz','20221223','EMAIL','ALPHA',0);
SELECT t1.ID, t1.DATE, t1.CHANNEL, t1.VENDOR, t1.ENG,
MIN(CASE WHEN t1.ENG = 0
THEN DATEDIFF(DAY, t_eng.DATE, t1.DATE)
ELSE 0
END) AS N_DAYS
FROM #t t1
LEFT OUTER JOIN #t t_eng ON t_eng.ENG = 1
AND t_eng.CHANNEL = t1.CHANNEL
AND t_eng.ID = t1.ID
AND t_eng.VENDOR = t1.VENDOR
AND t_eng.DATE < t1.DATE
GROUP BY t1.ID, t1.DATE, t1.CHANNEL, t1.VENDOR, t1.ENG

Answer 2

I was able to construct a right query that would give difference in days since last event in a chronological order with a massive help of James Casey. The query is as below:
QUERY:
SELECT T1.ID, T1.DATE, T1.CHANNEL, T1.VENDOR,
IFNULL(MIN(DATEDIFF(DD, T_ENG.EVENT_DATE, T1.EVENT_DATE)), -1) AS "N_DAYS"
FROM
TAB1 AS T1
LEFT OUTER JOIN
TAB1 AS T_ENG
ON T_ENG.ENG>0 AND T1.ID=T_ENG.ID AND T1.CHANNEL=T_ENG.CHANNEL AND T1.VENDOR=T_ENG.VENDOR
AND DATEDIFF(DD, T_ENG.DATE, T1.DATE)>=0
GROUP BY
T1.ID, T1.DATE, T1.CHANNEL, T1.VENDOR
ORDER BY 2;