How to turn this script into a UDF in BigQuery - google-bigquery

Question

This should be easy to do but I'm struggling.
I have the below script that calculates a number of business dates before or after a date. I need to change to UDF as I'll be using it in multiple views. It is for Bigquery:
DECLARE Date DATE;
DECLARE DAYS_EXTEND INT64;
DECLARE COUNTER INT64;
SET Date = '2021-12-23';
SET DAYS_EXTEND = 3;
SET COUNTER = DAYS_EXTEND;
BEGIN
WHILE COUNTER > 0 Do
SET COUNTER = COUNTER -1;
SET Date = Date +1;
IF Extract( DAYOFWEEK from Date) in (1,7) or Date in ('2021-01-01','2021-04-02','2021-04-05','2021-05-03','2021-05-31','2021-08-30','2021-12-27','2021-12-28','2022-01-03','2022-04-15','2022-04-18','2022-05-02','2022-06-02','2022-06-03','2022-08-29','2022-12-26','2022-12-27')
Then
BEGIN
SET DAYS_EXTEND = DAYS_EXTEND +1;
SET COUNTER = COUNTER +1;
END;
END IF;
END WHILE;
WHILE COUNTER <0 DO
SET COUNTER = COUNTER +1;
SET Date = Date -1;
IF Extract( DAYOFWEEK from Date) in (1,7) or Date in ('2021-01-01','2021-04-02','2021-04-05','2021-05-03','2021-05-31','2021-08-30','2021-12-27','2021-12-28','2022-01-03','2022-04-15','2022-04-18','2022-05-02','2022-06-02','2022-06-03','2022-08-29','2022-12-26','2022-12-27')
Then
BEGIN
SET DAYS_EXTEND = DAYS_EXTEND - 1;
SET COUNTER = COUNTER - 1;
END;
END IF;
END WHILE;
END;
I'm just not sure how to turn it into a Select statement or if it is possible to do UDF without a Select statement with the loops remaining.
I'd appreciate any help.

Answer 1

For what I understand from your question, you are trying to create a function/procedure that make uses of selects and loops to identify business days. Personally I would avoid complex scripting (usually as last resort). I think It can be achieve by just using selects.
Here is a sample code, that can assist you identifying business days:
DECLARE MyDate DATE;
DECLARE DAYS_RANGE INT64;
SET MyDate = '2021-11-22';
SET DAYS_RANGE = 10;
/*if it fits you, create a table for special dates and holidays*/
create temp table offdays (
nonworkingdays date
);
insert into offdays values('2021-11-23');
insert into offdays values('2021-11-19');
with working_dates as(
select dafter,
case when EXTRACT(DAYOFWEEK FROM dafter) not in (1,7) then 1 else 0 end as isweekdays_after,
case when dafter in (select nonworkingdays from offdays) then 1 else 0 end as isoffdays_after,
dbefore,
case when EXTRACT(DAYOFWEEK FROM dbefore) not in (1,7) then 1 else 0 end as isweekdays_before,
case when dbefore in (select nonworkingdays from offdays) then 1 else 0 end as isoffdays_before
from (
select date_add(MyDate, INTERVAL days_to_count DAY) as dafter,
date_add(MyDate, INTERVAL -days_to_count DAY) as dbefore
from (SELECT days_to_count FROM UNNEST(GENERATE_ARRAY(1, DAYS_RANGE)) AS days_to_count)
)
)
select * from working_dates
If you run it, you will get a main table working_dates with rows equals to the range given and columns that will help you identify weekdays and offdays.
So, You can use this code to create a function or procedure where you can pass parameters and calculate if you either want days after or before and return the days or the count of days filtered by the columns weekdays and offdays.
Take this as a sample function derived from above script:
CREATE TEMP FUNCTION GetBusinessDaysAfter(fnDate Date, days_range INT64)
RETURNS INT64
AS ((
select count(d.dafter) from(
select dafter,
case when EXTRACT(DAYOFWEEK FROM dafter) not in (1,7) then 1 else 0 end as isweekdays_after,
case when dafter in ('2021-11-23') then 1 else 0 end as isoffdays_after,
from (
select date_add(fnDate, INTERVAL days_to_count DAY) as dafter
from (SELECT days_to_count FROM UNNEST(GENERATE_ARRAY(1, days_range)) AS days_to_count)
)) as d
where d.isweekdays_after=1 and d.isoffdays_after=0
));
select GetBusinessDaysAfter('2021-11-22',3)
I can use it to retrieve how many business days will pop up in the next 3 days. Turns out to be just 2 (for the sake of the sample, I put a fixed value in offdays, you can replace it making a reference to your holidays table in your dataset).
For more information about scripting, functions and procedures, here are some useful links:
Data definition language
Working with arrays
Working with dates functions