How to find gap date and minimum date in the same query? - tsql

Question

I have a table customer_history which log customer_id and modification_date.
When customer_id is not modified there is no entry in the table
I can find when customer_id haven't been modified (=last_date_with_no_modification). I look for when the date is missing (= Gaps and Islands problem).
But in the same query if no date is missing the value last_date_with_no_modification should
be DATEADD(DAY,-1,min(modification_date)) for the customer_id.
I don't know how to add this last condition in my SQL query?
I use following tables:
"Customer_history" table:
customer_id modification_date
1 2017-12-20
1 2017-12-19
1 2017-12-17
2 2017-12-20
2 2017-12-18
2 2017-12-17
2 2017-12-15
3 2017-12-20
3 2017-12-19
"#tmp_calendar" table:
date
2017-12-15
2017-12-16
2017-12-17
2017-12-18
2017-12-19
2017-12-20
Query used to qet gap date:
WITH CTE_GAP AS
(SELECT ch.customer_id,
LAG(ch.modification_date) OVER(PARTITION BY ch.customer_id ORDER BY ch.modification_date) as GapStart,
ch.modification_date as GapEnd,
(DATEDIFF(DAY,LAG(ch.modification_date) OVER(PARTITION BY ch.customer_id ORDER BY ch.modification_date), ch.modification_date)-1) GapDays
FROM customer_history ch )
SELECT cg.customer_id,
DATEADD(DAY,1,MAX(cg.GapStart)) as last_date_with_no_modification
FROM CTE_GAP cg
CROSS JOIN #tmp_calendar c
WHERE cg.GapDays >0
AND c.date BETWEEN DATEADD(DAY,1,cg.GapStart) AND DATEADD(DAY,-1,cg.GapEnd)
GROUP BY cg.customer_id
Result:
customer_id last_date_with_no_modification
1 2017-12-18
2 2017-12-19
3 2017-12-19 (Row missing)
How to get customer_id 3?

Answer 1

Something this should work:
WITH CTE_GAP
AS
(
SELECT
ch.customer_id,
LAG(ch.modification_date) OVER(PARTITION BY ch.customer_id ORDER BY ch.modification_date) as GapStart,
ch.modification_date as GapEnd,
(DATEDIFF(DAY,LAG(ch.modification_date) OVER(PARTITION BY ch.customer_id ORDER BY ch.modification_date), ch.modification_date)-1) GapDays
FROM #customer_history ch
)
SELECT DISTINCT
C.customer_id
, ISNULL(LD.last_date_with_no_modification, LD_NO_GAP.last_date_with_no_modification) last_date_with_no_modification
FROM
customer_history C
LEFT JOIN
(
SELECT
cg.customer_id,
DATEADD(DAY, 1, MAX(cg.GapStart)) last_date_with_no_modification
FROM
CTE_GAP cg
CROSS JOIN #tmp_calendar c
WHERE
cg.GapDays >0
AND c.date BETWEEN DATEADD(DAY, 1, cg.GapStart) AND DATEADD(DAY, -1, cg.GapEnd)
GROUP BY cg.customer_id
) LD
ON C.customer_id = LD.customer_id
LEFT JOIN
(
SELECT
customer_id
, DATEADD(DAY, -1, MIN(modification_date)) last_date_with_no_modification
FROM customer_history
GROUP BY customer_id
) LD_NO_GAP
ON C.customer_id = LD_NO_GAP.customer_id