How to match the closet date in sql (redshift)? - sql

Question

For example, my table A is work_schedule:
Employee_id
Week_start
Work_schedule
A
2021-01-03
Day shift
A
2021-01-10
Day shift
A
2021-01-17
Night shift
B
2020-12-27
Day shift
B
2021-01-03
Day shift
Table B is employee_history:
Employee_id
Calendar_date
Tenure
A
2020-12-20
0
A
2020-12-21
1
A
---
2-30
A
2021-01-19
31
A
2021-01-20
32
B
2020-12-15
0
B
2020-12-16
1
B
---
Employee can choose work schedule 2 weeks ahead, and I want to fetch tenure at the snapshot date (2 weeks ahead). For employee A, the 14 days time period can match a calendar_date. But for employee B, he started within 2 weeks. I want to have the closet date to the 2-week date.
The ideal output is:
Employee_id
Week_start
Work_schedule
Calendar_date (to calculate tenure)
Tenure (at 2 weeks ago)
A
2021-01-03
Day shift
2020-12-20
0
A
2021-01-10
Day shift
2020-12-27
7
A
2021-01-17
Night shift
2021-01-03
14
B
2020-12-27
Day shift
2020-12-15
0
B
2021-01-03
Day shift
2020-12-20
5
For one record to fetch closet date, I can use
order by abs(datediff(day, (week_start - 14), calendar_date)) asc
limit 1
For example, fetch ‘2020-12-15’ as the closest date to ‘2020-12-13’.
select employee_id, calendar_date, tenure
from employee_history h
where employee_id = B
order by abs(datediff(day, ('2020-12-27' - 14), date_key)) asc
limit 1
But I have more than one employees in this situation, how can I get the closest calendar_date for all those that cannot find a match for exactly 2 weeks?