CRON expression to next DateTime with T-SQL on SQL server - sql-server

Question

I have a column that contains a CRON expression that represents how often a user has to perform a task. I would like to build a view that lists the date and things to do for a given user. But I need to calculate my next CRON occurrence in a datetime in T-SQL. How interpreted my expression CRON in SQL?
example:
column value = [0 30 8 1 *?]
I would write:
SELECT CrontabSchedule ( '0 30 8 1 *?', GETDATE ()) FROM dbo.UserTasks
Someone has a solution ?

Answer 1

There is no built in function within SQL server to genrate Next date based on Cron expression. Only way would be, implemeting cron exprssions in C# and genrate it as dll,then regsiter DLL with SQL server CLR.
Refernces:
https://social.msdn.microsoft.com/Forums/en-US/3e0ebe5a-d452-4893-bcef-0a1e2520c076/convert-cron-expression-to-datetime?forum=transactsql
https://github.com/atifaziz/NCrontab/wiki/SQL-Server-Crontab

Answer 2

Maybe this link can give you a hint: https://ask.sqlservercentral.com/questions/117285/tsql-sql-function-for-a-cron-expression.html
The UDF presented in the link is as follow:
CREATE PROCEDURE Q117285.CreateJobScheduleFromCronExpression
#CronExpression varchar(200)
, #FieldDelimiterCharacter char(1) -- any whitespace character
AS
BEGIN;
DECLARE #CronExpressionArray TABLE
(
Seconds varchar(50)
, Minutes varchar(50)
, Hours varchar(50)
, DayOfMonth varchar(50)
, Month varchar(50)
, DayOfWeek varchar(50)
, Year varchar(50)
);
INSERT #CronExpressionArray
SELECT
[Seconds] = [1]
, [Minutes] = [2]
, [Hours] = [3]
, [DayOfMonth] = [4]
, [Month] = [5]
, [DayOfWeek] = [6]
, [Year] = [7]
FROM
(
SELECT ds.ItemNumber, ds.Item
FROM dbo.DelimitedSplit8K(#CronExpression, #FieldDelimiterCharacter) ds
) pSrc
PIVOT
(
MAX(pSrc.Item)
FOR pSrc.ItemNumber IN
([1], [2], [3], [4], [5], [6], [7])
) pvt
;
SELECT * FROM #CronExpressionArray;
END;
The DelimitedSplit8K UDF was extracted from the article: http://www.sqlservercentral.com/articles/Tally+Table/72993/
CREATE FUNCTION [dbo].[DelimitedSplit8K]
--===== Define I/O parameters
(#pString VARCHAR(8000), #pDelimiter CHAR(1))
--WARNING!!! DO NOT USE MAX DATA-TYPES HERE! IT WILL KILL PERFORMANCE!
RETURNS TABLE WITH SCHEMABINDING AS
RETURN
--===== "Inline" CTE Driven "Tally Table" produces values from 1 up to 10,000...
-- enough to cover VARCHAR(8000)
WITH E1(N) AS (
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
), --10E+1 or 10 rows
E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows
E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max
cteTally(N) AS (--==== This provides the "base" CTE and limits the number of rows right up front
-- for both a performance gain and prevention of accidental "overruns"
SELECT TOP (ISNULL(DATALENGTH(#pString),0)) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4
),
cteStart(N1) AS (--==== This returns N+1 (starting position of each "element" just once for each delimiter)
SELECT 1 UNION ALL
SELECT t.N+1 FROM cteTally t WHERE SUBSTRING(#pString,t.N,1) = #pDelimiter
),
cteLen(N1,L1) AS(--==== Return start and length (for use in substring)
SELECT s.N1,
ISNULL(NULLIF(CHARINDEX(#pDelimiter,#pString,s.N1),0)-s.N1,8000)
FROM cteStart s
)
--===== Do the actual split. The ISNULL/NULLIF combo handles the length for the final element when no delimiter is found.
SELECT ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1),
Item = SUBSTRING(#pString, l.N1, l.L1)
FROM cteLen l
;

Answer 3

I needed to do something similar and eventually built my own tSQLcron
Currently it isn't SET based as it is a stored-procedure. An example usage is
DECLARE #out_is_cron_true BIT ;
EXEC tSQLcron.usp_is_date_in_cron_period
#cron_expression = N'* 0/15 * * * *' -- nvarchar(100)
, #validate_date = '2020-01-01 13:15:00' -- datetime
, #out_is_cron_true = #out_is_cron_true OUTPUT -- bit
IF (#out_is_cron_true = 1 )
BEGIN
PRINT 'DO SOMETHING';
END