Determine Days Not Covered Based on Fields Containing "From" and "To" Dates - sql-server

Question

Server: Microsoft SQL Server
SQLFiddle: http://www.sqlfiddle.com/#!18/cdfa3/1/0
If I have rows containing a "start" date and an "end date", how can write a SQL query that will list the days that are not contained between those dates.
Example (see SQLFiddle link above for a playable demo):
startdate enddate
2019-06-06 00:00:00.000 2019-06-08 00:00:00.000
2019-06-10 00:00:00.000 2019-06-11 00:00:00.000
2019-06-12 00:00:00.000 2019-06-13 00:00:00.000
We have a coverage gap on June 9th, because we have coverage from June 6th-June 8th, then on June 10th-June 13th.
How is it possible to identify the date of June 9th as having no coverage based on rows that have date ranges?

Answer 1

You could use generated calendar table and LEFT JOIN:
DECLARE #min DATE, #max DATE;
SELECT #min = MIN(workingdatestart), #max = MAX(workingdateend) FROM workingdates;
WITH cte AS (
SELECT DATEADD(DAY, ROW_NUMBER() OVER(ORDER BY 1/0), #min) AS d
FROM sys.objects s, sys.objects s2
)
SELECT c.d AS gap
FROM cte c
LEFT JOIN workingdates w ON c.d BETWEEN w.workingdatestart and w.workingdateend
WHERE c.d < #max AND w.workingDateId IS NULL;
db<>fiddle demo

Answer 2

#Lukasz Szozda stole my thunder. My answer is similar but does not use variables (I'm not suggesting that's good or bad.. just calling it out).
You can create a calendar table function (see example below) then perform a LEFT ANTI SEMI JOIN against your working days table. The benefit to this solution is the calendar table generates 0 IO.
Solution:
WITH r(L,H) AS
(
SELECT CAST(MIN(w.workingdatestart) AS DATE), CAST(MAX(w.workingdateend) AS DATE)
FROM dbo.workingdates AS w
),
cal AS
(
SELECT c.Dt
FROM r
CROSS APPLY dbo.calendar(r.L,r.H) AS c
)
SELECT c.Dt
FROM cal AS c
EXCEPT
SELECT c.Dt
FROM cal AS c
JOIN dbo.workingdates AS w
ON c.Dt BETWEEN w.workingdatestart AND w.workingdateend;
.. and the function:
CREATE FUNCTION dbo.calendar(#startdate DATE, #enddate DATE)
RETURNS TABLE WITH SCHEMABINDING AS RETURN
WITH E1(N) AS (SELECT 1 FROM (VALUES (1),(1),(1),(1),(1),(1),(1),(1),(1),(1)) AS x(x)),
iTally(N) AS
(
SELECT 0 UNION ALL
SELECT TOP (DATEDIFF(DAY,#startDate,#endDate)) ROW_NUMBER() OVER (ORDER BY (SELECT 1))
FROM E1 a, E1 b, E1 c
)
SELECT sortKey = i.N, Dt = DATEADD(DAY, i.N, #startDate)
FROM iTally AS i;

Answer 3

To achieve this you need a table that contains all dates between your minimal and maximal dates. Then just filter rows that don't exist in workingdates using LEFT JOIN.
declare #minDate date = (select min([workingdatestart]) from [workingdates])
declare #maxDate date = (select max([workingdateend]) from [workingdates])
declare #Date date = #minDate
create table #rangeOfDates (dat date)
while #Date <= #maxDate
begin
insert into #rangeOfDates values (#Date)
set #Date = dateadd(day , 1, #Date)
end
select r.dat
from #rangeOfDates as r
left join workingdates as w
on r.dat between w.workingdatestart and w.workingdateend
where w.workingdateID is null
Result:
dat
2019-06-09