ISDATE Function for different date formats in TSQL - sql-server

Question

I need to convert VARCHAR values into DATETIME in multiple columns of a view for sorting and formatting (displaying in locale format) purposes in another application on SQL Server 2008.
There are currently two problems.
The input format of the VARCHAR values differ (but consistent at
column level)
Also there may be faulty values (e.g.: 20..05.2015)
Unfortunately the TRY_CONVERT function is available just for SQL Server 2012 and later.
ISDATE does not work because the view contains different date formats and I can neither set the language inside user defined functions nor in views, which would cause ISDATE to work with german date formats for example.
Is there any easier solution for my problem?
My first thought was to write a function like
FUNCTION TryConvertStringAsDatetime ( #value VARCHAR(MAX),
#format INT
)
that uses the format numbers of the CONVERT function, but checking for every possible format manually scares me a bit.
Example: TryConvertStringAsDatetime('20.05.2015', 104) (with some pseudocode)
SET #day = character 1 and 2
SET #month = character 4 and 5
SET #year = character 7, 8, 9 and 10
SET #dateODBCFormat = #year - #month - #day (concatenated with hyphen and not subtracted :)
IF ISDATE(#dateODBCFormat ) = 1
RETURN CONVERT(DATETIME, #dateODBCFormat, 120)
ELSE
RETURN CONVERT(DATETIME, 0) (does the job)

Answer 1

This is the function I now came up with:
CREATE
FUNCTION TryConvertStringAsDatetime ( #value VARCHAR(MAX),
#format INT
)
RETURNS DATETIME
AS
/*
Tries to convert a given VARCHAR value to DATETIME.
Returns NULL if no value was specified or the value is not in the correct format.
*/
BEGIN
DECLARE #length INT = LEN(#value)
IF #length IS NULL OR #length < 10 OR #length > 23
RETURN NULL
DECLARE #day VARCHAR(2),
#month VARCHAR(2),
#year VARCHAR(4),
#time VARCHAR(9)
IF #format = 104 --dd.mm.yyyy hh:mi:ss(24h)
BEGIN
SET #day = SUBSTRING(#value, 1, 2)
SET #month = SUBSTRING(#value, 4, 2)
SET #year = SUBSTRING(#value, 7, 4)
END
ELSE IF #format IN (120, 121) --yyyy-mm-dd hh:mi:ss(24h)
BEGIN
SET #year = SUBSTRING(#value, 1, 4)
SET #month = SUBSTRING(#value, 6, 2)
SET #day = SUBSTRING(#value, 9, 2)
END
ELSE
RETURN NULL -- currently only german and ODBC supported
IF #length > 11
SET #time = SUBSTRING(#value, 12, #length - 11)
SET #value = #year + '-' + #month + '-' + #day + ISNULL(' ' + #time, '')
IF ISDATE(#value) = 1
RETURN CONVERT(DATETIME, #value, 121)
RETURN NULL
END

Answer 2

I would probably go with something like this:
CREATE FUNCTION TryConvertToDate
(
#InputString varchar(20)
)
RETURNS Datetime
BEGIN
DECLARE #DateTime datetime = NULL
SET #DateTime =
CASE
WHEN LEN(#InputString) = 10 AND PATINDEX('[0-9][0-9].[0-9][0-9].[0-9][0-9][0-9][0-9]', #InputString)=1 THEN
CONVERT(DateTime, #InputString, 104) -- German
WHEN LEN(#InputString) = 10 AND PATINDEX('[0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]', #InputString)=1 THEN
CONVERT(DateTime, #InputString, 120) -- ODBC
ELSE
NULL -- unsuported format
END
RETURN #DateTime
END
Note: Testing for length and using patindex ensures only general format, so you need the call this function inside a try block in case the days and months are inverted and will cause a conversion error.
On the other hand, adding supported formats to this function is very easy - all you have to do is add a when clause with the correct patindex and length and the correct convert style.
Another option is to ensure the string can actually be converted to date.
This will make your function more complicated and thus harder to write, but will be easier to work with as it will reduce to minimum the chance of raising a conversion error:
CREATE FUNCTION TryConvertToDate
(
#InputString varchar(20)
)
RETURNS Datetime
BEGIN
DECLARE #DateValue date, #Days int, #Months int, #Years int
IF LEN(#DateString) = 10 AND PATINDEX('[0-9][0-9].[0-9][0-9].[0-9][0-9][0-9][0-9]', #InputString)=1 -- German format
BEGIN
SELECT #Days = CAST(LEFT(#InputString, 2) As int),
#Months = CAST(SUBSTRING(#InputString, 4, 2) as int),
#Years = CAST(RIGHT(#InputString, 4) as int)
-- NOTE: you will need to add a condition for leap years
IF (#Days < 31 AND #Months IN(4,6,9,12)) OR (#Days < 30 AND #Months = 2)
SET #DateValue = convert(date, #InputString, 104)
END
IF LEN(#InputString) = 10 AND PATINDEX('[0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]', #InputString)=1 -- ODBC format
BEGIN
SELECT #Days = CAST(RIGHT(#InputString, 2) As int),
#Months = CAST(SUBSTRING(#InputString, 6, 2) as int),
#Years = CAST(LEFT(#InputString, 4) as int)
-- NOTE: you will need to add a condition for leap years
IF (#Days < 31 AND #Months IN(4,6,9,12)) OR (#Days < 30 AND #Months = 2)
SET #DateValue = convert(date, #InputString, 120)
END
RETURN #DateValue
END

Answer 3

You might have better luck in terms of both speed and functionality doing this in SQLCLR (as noted by #Tab and #Zohar in various comments).
.NET / C# code:
using System;
using System.Data.SqlTypes;
using System.Globalization;
using Microsoft.SqlServer.Server;
public class TryConvertStuff
{
[SqlFunction(IsDeterministic = true, IsPrecise = true)]
public static SqlDateTime TryConvertDateTime([SqlFacet(MaxSize = 50)] SqlString
StringDate, [SqlFacet(MaxSize = 10)] SqlString Culture)
{
CultureInfo _Culture = CultureInfo.CurrentCulture;
if (!Culture.IsNull && Culture.Value.Trim() != String.Empty)
{
_Culture = CultureInfo.GetCultureInfo(Culture.Value);
}
DateTime _RealDate;
if (DateTime.TryParse(StringDate.Value, _Culture,
DateTimeStyles.None, out _RealDate))
{
return _RealDate;
};
return SqlDateTime.Null;
}
}
Tests:
SELECT dbo.TryConvertDateTime(N'2019-04-20', N'en'); -- 2019-04-20 00:00:00.000
SELECT dbo.TryConvertDateTime(N'2019-04-20f', N'en'); -- NULL
SELECT dbo.TryConvertDateTime(N'2019.04.20', N'en'); -- 2019-04-20 00:00:00.000
SELECT dbo.TryConvertDateTime(N'20.04.2019', N'en'); -- NULL
SELECT dbo.TryConvertDateTime(N'20.04.2019', N'de'); -- 2019-04-20 00:00:00.000
SELECT dbo.TryConvertDateTime(N'20.04.2019', NULL); -- NULL