SQL Server 2017

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CREATE TABLE zz_position  
(
     posNo varchar(25) NOT NULL,  
     description varchar(25) NOT NULL,  
     date_from DATE NULL,  
     date_to DATE NULL
)

insert into zz_position 
values ('10001', 'System Administrator', '2017-01-01 00:00:00.000', '2020-12-31 00:00:00.000')

CREATE TABLE zz_resource
(
     resID varchar(25) NOT NULL,  
     description varchar(25) NOT NULL,  
     date_from DATE NULL,  
     date_to DATE NULL
)

insert into zz_resource 
values ('100', 'Sam', '2016-01-01 00:00:00.000', '2099-12-31 00:00:00.000'),
       ('101', 'Joe', '2015-01-01 00:00:00.000', '2099-12-31 00:00:00.000')

CREATE TABLE zz_employment
(
      posNo varchar(25) NOT NULL,  
      resID varchar(25) NOT NULL,  
      date_from DATE NULL,  
      date_to DATE NULL,
      seqNo int NULL
)

insert into zz_employment 
values ('10001', '100', '2018-01-01 00:00:00.000', '2018-04-01 00:00:00.000', 1),
       ('10001', '101', '2018-06-01 00:00:00.000', '2019-12-31 00:00:00.000', 2),
       ('10001', '100', '2020-01-01 00:00:00.000', '2099-12-31 00:00:00.000', 3)

DECLARE @DateStart DATE = (SELECT MIN(P.date_from) FROM zz_position AS P)
DECLARE @DateEnd DATE = (SELECT(MAX(P.date_to)) FROM zz_position AS P)

;WITH GeneratedDates AS
(
	SELECT
		GeneratedDate = @DateStart
	
	UNION ALL
	
	SELECT
		GeneratedDate = DATEADD(DAY, 1, G.GeneratedDate)
	FROM
		GeneratedDates AS G
	WHERE
		DATEADD(DAY, 1, G.GeneratedDate) <= @DateEnd
)
SELECT
	DateID = IDENTITY(INT, 1, 1),
	G.GeneratedDate
INTO
	Calendar
FROM
	GeneratedDates AS G
OPTION
	(MAXRECURSION 0)

;WITH AllPositionDates AS
(
	SELECT
		T.posNo,
		C.GeneratedDate
	FROM
		zz_position AS T
		INNER JOIN Calendar AS C ON C.GeneratedDate BETWEEN T.date_from AND T.date_to
),
AllEmployedDates AS
(
	SELECT
		T.posNo,
		T.resID,
		T.seqNo,
		C.GeneratedDate
	FROM
		zz_employment AS T
		INNER JOIN Calendar AS C ON C.GeneratedDate BETWEEN T.date_from AND T.date_to
),
PositionsByEmployed AS
(
	SELECT
		P.posNo,
		P.GeneratedDate,
		E.resID,
		E.seqNo,
		NullRowNumber = ROW_NUMBER() OVER (
			PARTITION BY
				P.posNo,
				CASE WHEN E.posNo IS NULL THEN 1 ELSE 2 END
			ORDER BY
				P.GeneratedDate ASC)
	FROM
		AllPositionDates AS P
		LEFT JOIN AllEmployedDates AS E ON
			P.posNo = E.posNo AND
			P.GeneratedDate = E.GeneratedDate
)
SELECT
	P.posNo,
	P.resID,
	Date_From = MIN(P.GeneratedDate),
	Date_To = MAX(P.GeneratedDate),
	seqNo = ISNULL(P.seqNo, 0)
FROM
	PositionsByEmployed AS P
GROUP BY
	P.posNo,
	P.resID,
	P.seqNo,
	P.NullRowNumber - DATEDIFF(DAY, '2000-01-01', P.GeneratedDate) -- GroupingValue
ORDER BY
	P.posNo,
	Date_From

posNo	resID	Date_From	Date_To	seqNo
10001	null	2017-01-01	2017-12-31	0
10001	100	2018-01-01	2018-04-01	1
10001	null	2018-04-02	2018-05-31	0
10001	101	2018-06-01	2019-12-31	2
10001	100	2020-01-01	2020-12-31	3