Minimum and maximum dates within continuous date range grouped by name - sql

Question

I have a data ranges with start and end date for a persons, I want to get the continuous date ranges only per persons:
Input:
NAME | STARTDATE | END DATE
--------------------------------------
MIKE | **2019-05-15** | 2019-05-16
MIKE | 2019-05-17 | **2019-05-18**
MIKE | 2020-05-18 | 2020-05-19
Expected output like:
MIKE | **2019-05-15** | **2019-05-18**
MIKE | 2020-05-18 | 2020-05-19
So basically output is MIN and MAX for each continuous period for the person.
Appreciate any help.
I have tried the below query:
With N AS ( SELECT Name, StartDate, EndDate
, LastStop = MAX(EndDate)
OVER (PARTITION BY Name ORDER BY StartDate, EndDate
ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING) FROM Table ), B AS ( SELECT Name, StartDate, EndDate
, Block = SUM(CASE WHEN LastStop Is Null Then 1
WHEN LastStop < StartDate Then 1
ELSE 0
END)
OVER (PARTITION BY Name ORDER BY StartDate, LastStop) FROM N ) SELECT Name
, MIN(StartDate) DateFrom
, MAX(EndDate) DateTo FROM B GROUP BY Name, Block ORDER BY Name, Block
But its not considering the continuous period. It's showing the same input.

Answer 1

This is a type of gap-and-islands problem. There is no need to expand the data out by day! That seems very inefficient.
Instead, determine the "islands". This is where there is no overlap -- in your case lag() is sufficient. Then a cumulative sum and aggregation:
select name, min(startdate), max(enddate)
from (select t.*,
sum(case when prev_enddate >= dateadd(day, -1, startdate) then 0 else 1 end) over
(partition by name order by startdate) as grp
from (select t.*,
lag(enddate) over (partition by name order by startdate) as prev_enddate
from t
) t
) t
group by name, grp;
Here is a db<>fiddle.

Answer 2

Here is an example using an ad-hoc tally table
Example or dbFiddle
;with cte as (
Select A.[Name]
,B.D
,Grp = datediff(day,'1900-01-01',D) - dense_rank() over (partition by [Name] Order by D)
From YourTable A
Cross Apply (
Select Top (DateDiff(DAY,StartDate,EndDate)+1) D=DateAdd(DAY,-1+Row_Number() Over (Order By (Select Null)),StartDate)
From master..spt_values n1,master..spt_values n2
) B
)
Select [Name]
,StartDate= min(D)
,EndDate = max(D)
From cte
Group By [Name],Grp
Returns
Name StartDate EndDate
MIKE 2019-05-15 2019-05-18
MIKE 2020-05-18 2020-05-19
Just to help with the Visualization, the CTE generates the following

Answer 3

This will give you the same result
SELECT subquery.name,min(subquery.startdate),max(subquery.enddate1)
FROM (SELECT NAME,startdate,
CASE WHEN EXISTS(SELECT yt1.startdate
FROM t yt1
WHERE yt1.startdate = DATEADD(day, 1, yt2.enddate)
) THEN null else yt2.enddate END as enddate1
FROM t yt2) as subquery
GROUP by NAME, CAST(MONTH(subquery.startdate) AS VARCHAR(2)) + '-' + CAST(YEAR(subquery.startdate) AS VARCHAR(4))
For the CASE WHEN EXISTS I refered to SQL CASE
For the group by month and year you can see this GROUP BY MONTH AND YEAR
DB_FIDDLE