Converting text to Datetime in specific format - sql-server

Question

I need an expression which will convert a text (VARCHAR) column to a DATETIME if, and only if, it matches dd/MM/yyyy, d/MM/yyyy, dd/M/yyyy or d/M/yyyy. If it doesn't match then I want a NULL.
I have this...
CASE ISDATE([DateField])
WHEN 1 THEN CONVERT(DATETIME,[DateField],103)
ELSE NULL
END
However this fails for '15/04/76' for example - with a "Conversion failed when converting datetime from character string" error - whereas I would want it to return NULL
Example output
'1/6/1976' -> 1976-06-01
'01/06/1976' -> 1976-06-01
'13/06/2001' -> 2001-06-13
'06/13/2001' -> NULL
'13/06/76' -> NULL
Is there a way of forcing ISDATE to validate a given format?
The documentation seems to suggest so...
ISDATE is deterministic only if used with the CONVERT function, the
CONVERT style parameter is specified and style is not equal to 0, 100,
9, or 109.
But ISDATE only takes one argument, so how do I "use it with CONVERT function" if I am not doing so already?

Answer 1

You could do a nested case statement here. The first could check to see if you have a 10 character string 2 for day, 2 for month, 4 for year and 2 for separators = 10 characters.
SET DATEFORMAT DMY;
Case When DateField Like '%/%/[0-9][0-9][0-9][0-9]'
Then Case When IsDate(DateField) = 1
Then CONVERT(DATETIME,[DateField],103)
End
End
Revised: I changed the code to use a like search which forces there to be a /YYYY at the end of the string, and then does an IsDate check to allow for a single day and/or month.

Answer 2

Well, first off, why on earth are you storing datetime values in a varchar column? This is a cardinal sin for a variety of reasons, not the least of which is that you get no validation whatsoever that the data is (or is convertible to) a datetime. You should also consider validating the input, even if you leave the column as varchar, so you don't have such a wide variety of potential formats that you want to consider valid.
So here is one way, borrowing a bit from #G Mastros:
DECLARE #f TABLE(i INT, d VARCHAR(32));
INSERT #f VALUES
(1,'15/04/76'),
(2,'15/04/1976'),
(3,'1/3/1976'),
(4,'1/3/76'),
(5,'15/3/1976'),
(6,'22/22/22'),
(7,'Yesterday');
SET DATEFORMAT DMY;
SELECT i, d, d2 = CASE WHEN ISDATE(d) = 1
AND d LIKE '%/[0-9][0-9][0-9][0-9]'
THEN CONVERT(DATETIME, d, 103) END
FROM #f;
Results:
i d d2
- ---------- -----------------------
1 15/04/76 NULL
2 15/04/1976 1976-04-15 00:00:00.000
3 1/3/1976 1976-03-01 00:00:00.000
4 1/3/76 NULL
5 15/3/1976 1976-03-15 00:00:00.000
6 22/22/22 NULL
7 Yesterday NULL
PS this will be a great case for TRY_CONVERT in SQL Server 2012. It does exactly what you're asking - it tries to convert to the specified data type; if it can't, it returns NULL.

Answer 3

Thanks for the responses folks.
I've done it like this...
CASE ISDATE([DateField]) WHEN 1 THEN
CASE WHEN SUBSTRING([DateField],LEN([DateField])-4,1) = '/' THEN
CASE WHEN CHARINDEX('/',[DateField],LEN([DateField])-3)=0 THEN
CONVERT(datetime, [DateField] , 103)
END
END
END
which is pretty nasty business so would still appreciate something neater!
But this doesn't work either - it still errors on mm/dd/yyyy format dates!
Scrap that last comment - it does seem to work now? Probably something to do with SET DATEFORMAT