Question

While converting and normalizing data from an Access database to a SQL Server database, I ran across an interesting issue: ISDATE() does not appear to successfully predict whether text can be cast to a date.

SELECT 'This will return an error' =
    CASE
    WHEN ISDATE('1/1-2010') = 1 THEN CAST('1/1-2010' AS date)
    ELSE NULL
    END

I thought this was a bit odd, as the ISDATE() MSDN article states that the function will return “1 if the expression is a valid date, time, or datetime value; otherwise, 0.”

From what I could tell, this error is not mitigated by playing with either SET DATEFORMAT or SET LANGUAGE.

Nor does CONVERT() appear to have a different result than CAST().

Fortunately, ISDATE() does successfully predict whether text can be converted to a datetime.

SELECT 'This will return a datetime' =
    CASE
    WHEN ISDATE('1/1-2010') = 1 THEN CAST('1/1-2010' AS datetime)
    ELSE NULL
    END

So, I could use a workaround of casting to a datetime and then casting to a date.

SELECT 'This will return a date' =
    CASE
    WHEN ISDATE('1/1-2010') = 1 THEN CAST(CAST('1/1-2010' AS datetime) as date)
    ELSE NULL
    END

However, I wonder if I am missing something. Is there a cleaner way to cast text to a date? As a corollary, is the ISDATE() function not defined correctly on MSDN (and actually therefore a bit of a misnomer)?

Answer 1

Neither IsDate nor IsNumeric actually attempt to convert your value to the type in question. Instead, they tell you if the value in question looks like it could be converted to a date or number. SQL Server 2012 has addressed this with the TRY_PARSE function. In the interim, you need to do some cleanup on the data or create a function that does that cleanup.