update a column with the latest hiredate of an employee - tsql

Question

I have an employee status history table.
I need to create one more column that should copy the min(EffectiveStartDate) on each row till the employee is rehired. I need to get the length of service of the employee where the date will be passed by UI.
How can i achieve in SQL server 2014

Answer 1

This answer has a few assumptions.
Assumptions
The data set is only for one Employee at a time. If it is not,
and there is another column, such as EmployeeID, then you will
want to specify that in a partition by clause inside the over
clause where my comments denote that.
That the EmployeeStatusCatalog values have the below meanings:
A: Active
L: Leave (of Absence)
I: Inactive
That a "Hire" or "Rehire" transaction is considered to happen
either at initial A status, or after an I status has ended.
Sample Data Setup
Did not include the EmployeeStatusId column, as my assumption is that it is not relevant to creating the expected outcome.
declare #employee table
(
EffectiveStartDate date not null
, EffectiveEndDate date not null
, EmployeeStatusCatalog char(1) not null
)
insert into #employee
values ('2008-02-29', '2016-05-31', 'A')
, ('2016-06-01', '2016-06-30', 'A')
, ('2016-07-01', '2016-07-30', 'L')
, ('2016-07-31', '2016-09-02', 'A')
, ('2016-09-03', '2016-10-09', 'I')
, ('2016-10-10', '2016-11-01', 'A')
, ('2016-11-02', '2016-12-02', 'L')
, ('2016-12-03', '2016-12-05', 'I')
, ('2016-12-06', '2016-12-06', 'A')
, ('2016-12-07', '2017-01-01', 'L')
, ('2017-01-02', '9999-12-31', 'A')
Answer
As you may or may not know, this is a classic gaps and islands scenario. Where each segment between Hire/Rehire dates is an island (no gaps in this example).
I used a CTE to move the I status forward one row (via LAG function), and then get the running count of the number of I rows to give each island a "ID" number.
After that, used a min function, while partitioning by the island number, to determine the minimum EffectiveStartDate for each island.
; with inactive_dts as
(
--move the I status forward one row
select e.EffectiveStartDate
, e.EffectiveEndDate
, e.EmployeeStatusCatalog
, lag(e.EmployeeStatusCatalog, 1, 'A') over (/*partion by here*/ order by e.EffectiveStartDate asc) as prev_status
from #employee as e
where 1=1
)
, active_island_nbr as
(
--get the running count of the number of I rows
select a.EffectiveStartDate
, a.EffectiveEndDate
, a.EmployeeStatusCatalog
, a.prev_status
, sum(case a.prev_status when 'I' then 1 else 0 end) over (/*partition by here*/ order by a.EffectiveStartDate asc) as ActiveIslandNbr
from inactive_dts as a
)
select min(a.EffectiveStartDate) over (partition by a.ActiveIslandNbr) as HireRehireDate
, a.EffectiveStartDate
, a.EffectiveEndDate
, a.EmployeeStatusCatalog
from active_island_nbr as a
Results
HireRehireDate EffectiveStartDate EffectiveEndDate EmployeeStatusCatalog
2008-02-29 2008-02-29 2016-05-31 A
2008-02-29 2016-06-01 2016-06-30 A
2008-02-29 2016-07-01 2016-07-30 L
2008-02-29 2016-07-31 2016-09-02 A
2008-02-29 2016-09-03 2016-10-09 I
2016-10-10 2016-10-10 2016-11-01 A
2016-10-10 2016-11-02 2016-12-02 L
2016-10-10 2016-12-03 2016-12-05 I
2016-12-06 2016-12-06 2016-12-06 A
2016-12-06 2016-12-07 2017-01-01 L
2016-12-06 2017-01-02 9999-12-31 A