Add business days to another table using a date table in SQL - sql

Question

I have these 2 tables with values:
create table FieldTable (
FieldId int,
FieldName varchar(256),
FieldValue sql_variant);
go
insert into FieldTable values (1, 'ABC', '04/17/2020');
go
create table DatbeTable (
Dt date,
DayType char);
go
insert into DatbeTable values ('2020-04-01','B');
insert into DatbeTable values ('2020-04-02','B');
insert into DatbeTable values ('2020-04-03','B');
insert into DatbeTable values ('2020-04-04','W');
insert into DatbeTable values ('2020-04-05','W');
insert into DatbeTable values ('2020-04-06','B');
insert into DatbeTable values ('2020-04-07','B');
insert into DatbeTable values ('2020-04-08','B');
insert into DatbeTable values ('2020-04-09','B');
insert into DatbeTable values ('2020-04-10','B');
insert into DatbeTable values ('2020-04-11','W');
insert into DatbeTable values ('2020-04-12','W');
insert into DatbeTable values ('2020-04-13','B');
insert into DatbeTable values ('2020-04-14','B');
insert into DatbeTable values ('2020-04-15','B');
insert into DatbeTable values ('2020-04-16','B');
insert into DatbeTable values ('2020-04-17','B');
insert into DatbeTable values ('2020-04-18','W');
insert into DatbeTable values ('2020-04-19','W');
insert into DatbeTable values ('2020-04-20','B');
insert into DatbeTable values ('2020-04-21','B');
insert into DatbeTable values ('2020-04-22','B');
insert into DatbeTable values ('2020-04-23','B');
insert into DatbeTable values ('2020-04-24','B');
insert into DatbeTable values ('2020-04-25','W');
insert into DatbeTable values ('2020-04-26','W');
insert into DatbeTable values ('2020-04-27','B');
insert into DatbeTable values ('2020-04-28','B');
insert into DatbeTable values ('2020-04-29','B');
insert into DatbeTable values ('2020-04-30','B');
go
I want to add 3 business days from the FieldValue date to the new column: NewFieldValue for that row.
select
FieldId,
FieldName,
FieldValue--,
--NewFieldValue
into #temp
from FieldTable ft
join DatbeTable d
on format(try_convert(date, ft.FieldValue), 'd', 'en-us') = format(try_convert(date, d.Dt), 'd', 'en-us');
go
I can't really figure out how to use the 2 tables to get this below result:
FieldId FieldName FieldValue NewFieldValue
------- --------- ---------- -------------
1 ABC 04/17/2020 04/22/2020

Answer 1

You can use row_number() to find out the nth business day from your base date
select
FieldId,
FieldName,
FieldValue,
d.Dt as NewFieldValue
from FieldTable ft
cross apply
(
select x.Dt, day_no = row_number() over (ORDER BY x.Dt)
from DatbeTable x
where x.Dt > try_convert(date, ft.FieldValue)
and x.DayType = 'B' -- business day only
) d
where d.day_no = 3 -- add 3 business days
Note : Don't convert the date to string for comparison.
PS : you should explain what is B and W. Thought it is pretty obvious that it is Business and Weekend but nevertheless, it is clearer if explain the related columns in your table.