Finding original start date to absence period - sql

Question

Could you please take a look at the following task?
I have DATA table (it contains data for previous week):
CREATE TABLE DATA
(
EMPLOYEE nvarchar(50),
ABSENCE_START_DATE datetime,
ABSENCE_END_DATE datetime,
ABSENCE_TYPE nvarchar(50)
)
ABSENCE_START_DATE - date when absence starts
ABSENCE_END_DATE - date when absence ends
ABSENCE_TYPE - type of absence
Current table contains the following data:
INSERT INTO DATA(EMPLOYEE,ABSENCE_START_DATE,ABSENCE_END_DATE,ABSENCE_TYPE) VALUES
('EMP01','2017-09-04 00:00:00.000','2017-09-06 00:00:00.000','Sickness'),--Monday - Wednesday
('EMP01','2017-09-08 00:00:00.000','2017-09-08 00:00:00.000','Vacation'),--Friday - Friday
('EMP02','2017-09-04 00:00:00.000','2017-09-09 00:00:00.000','Sickness'),--Monday - Friday
('EMP03','2017-09-05 00:00:00.000','2017-09-09 00:00:00.000','Sickness')--Tuesday - Friday
Also, I have another table - STORAGE (it contains data for dates which are earlier than start of previous week).
CREATE TABLE STORAGE
(
EMPLOYEE nvarchar(50),
APPLY_DATE datetime,
ABSENCE_TYPE nvarchar(50)
)
There are daily records (excluding Saturdays and Sundays - they will never exist in this table)
INSERT INTO STORAGE(EMPLOYEE,APPLY_DATE,ABSENCE_TYPE) VALUES
('EMP01','2017-08-27 00:00:00.000','Sickness'),
('EMP01','2017-08-28 00:00:00.000','Worked'),
('EMP01','2017-08-29 00:00:00.000','Worked'),
('EMP01','2017-08-30 00:00:00.000','Sickness'),
('EMP01','2017-08-31 00:00:00.000','Sickness'),
('EMP01','2017-09-01 00:00:00.000','Sickness'),
('EMP02','2017-08-31 00:00:00.000','Worked'),
('EMP02','2017-09-01 00:00:00.000','Sickness')
So, the task is:
sql -script should find original start date to absence periods (from DATA table) which absence start date is Monday.
In other words, script should go day after day "in the past" and find date when appropriate absence period starts.
Not necessary that absence on Monday is 'Sickness'. It could be also 'Travel','Maternity'...
Expected result for examples below is (pay attention to first and third rows - absence start dates are different from appropriate rows in DATA table):
Thank you in advance.

Answer 1

From the sample data shared, it seems that you are looking to retrieve min date from storage to absence_start_date column of data table, below query can be an option.
SELECT d.Employee,
coalesce(CASE
WHEN d.ABSENCE_TYPE = 'Sickness' THEN
(SELECT min(apply_date)
FROM
STORAGE s
WHERE s.employee = d.employee
AND s.ABSENCE_TYPE = 'Sickness')
ELSE d.ABSENCE_START_DATE
END,d.ABSENCE_START_DATE) AS ABSENCE_START_DATE,
d.ABSENCE_END_DATE,
ABSENCE_TYPE
FROM DATA d;
Update 1:
A more generic query is below.
SELECT d.Employee,
coalesce(
(SELECT min(apply_date)
FROM
STORAGE s
WHERE s.employee = d.employee
AND s.ABSENCE_TYPE = d.ABSENCE_TYPE),d.ABSENCE_START_DATE) AS ABSENCE_START_DATE,
d.ABSENCE_END_DATE,
d.ABSENCE_TYPE
FROM DATA d
Update 2:
If you want to exclude weekends from the data, below is the query.
SELECT d.Employee,
coalesce(
(SELECT min(apply_date)
FROM
STORAGE s
WHERE s.employee = d.employee
AND s.ABSENCE_TYPE = d.ABSENCE_TYPE
AND DATENAME(dw,apply_date) NOT IN('Sunday','Saturday')),d.ABSENCE_START_DATE) AS ABSENCE_START_DATE,
d.ABSENCE_END_DATE,
d.ABSENCE_TYPE
FROM DATA d
Result:
Employee ABSENCE_START_DATE ABSENCE_END_DATE ABSENCE_TYPE
--------------------------------------------------------------------
EMP01 30.08.2017 00:00:00 06.09.2017 00:00:00 Sickness
EMP01 08.09.2017 00:00:00 08.09.2017 00:00:00 Vacation
EMP02 01.09.2017 00:00:00 09.09.2017 00:00:00 Sickness
EMP03 05.09.2017 00:00:00 09.09.2017 00:00:00 Sickness
you can check the demo here
Hope this will help.