Validate datetime inside a t-sql function - sql

Question

I need to validate a date inside a social security number checking function. The date format it should accept is ddmmyyyy, for example 01012000 (first of Jan, 2000).
I have tried converting this string into yyyy-mm-dd, and using IsDate() function, but the problem is that sometimes the datetimeformat of SqlServer might be something else like dmy, and I cannot use the "set DateFormat ymd" inside a function because it gives this error:
Invalid use of a side-effecting operator 'SET COMMAND' within a function.
Also tried a try .. catch block but with the same results.
So I need a way to validate datetime, regardless of the SqlServer dateformat, in a user function.

Answer 1

This snippet of code will try to convert the input. It first changes the string to a German time format dd.mm.yyyy with convert format 104. It uses begin try to suppress any error during the conversion.
If it succeeds, the date is in #output_dt. If it fails, #output_dt will be null.
declare #input_str varchar(25)
declare #output_dt date
set #input_str = '31122011'
if len(#input_str) >= 8
begin
declare #german_str varchar(25)
select #german_str = substring(#input_str, 1, 2) + '.' +
substring(#input_str, 3, 2) + '.' +
substring(#input_str, 5, 4)
begin try
select #output_dt = convert(date, #german_str, 104)
end try
begin catch
end catch
end
select #output_dt
If you can't use begin catch, you could create a table with valid dates. This example creates a table called ValidDates with dates from 1900 to 2100:
if object_id('ValidDates') is not null
drop table ValidDates
create table ValidDates (date_str varchar(8), dt date)
go
truncate table ValidDates
declare #start_dt date
declare #end_dt date
set #start_dt = '1900-01-01'
set #end_dt = '2100-01-01'
; with cte as
(
select #start_dt as dt
union all
select dateadd(day, 1, dt)
from cte
where dt < #end_dt
)
insert ValidDates
(date_str, dt)
select replace(convert(varchar(10), dt, 104),'.','')
, dt
from cte
option (maxrecursion 0);
You can then check for a valid date like:
select #output_dt = dt from ValidDates where date_str = #input_dt

Answer 2

Seems like
SELECT ISDATE('2005-05-22') is affected by SET DATEFORMAT dmy and returns zero
but
SELECT ISDATE('20050522') is not affected even when dateformat is dmy, so this one can be used independently from any dateformat used as default by database.