Error catching a function error in TSQL - tsql

Question

I am working with a SQL Server 2005 database table that is currently storing dates as varchars. This is outside of my control. For ease of reporting, I would like to create a view that converts these varchar dates to datetime fields.
The varchar dates are formatted for the most part, except for the occasional typing error.
DateString
----------
2001/01/02 -- most of the fields
2002/0601 -- typo, missing slash between month and day
2004/02/30 -- typo, no 30th of February
Because the dates are already formatted, I'm using the cast function to convert them to datetime.
cast(DateString as datetime)
The problem is when the cast function comes across an incorrect date, the query ends in error.
Is there are way to wrap just the function in a try...catch block? I see it is possible to wrap an entire query in a try...catch block, but the full query has multiple casts that must be done, and any combination could have typing errors.

Answer 1

I would use the built-in ISDATE() function. You can then write a CASE statement within your SELECT statement to either return the parsed date or either a null or some other result. Or, you can place it directly in the WHERE clause to only return those rows where there is a valid date.
A possible solution with the ISDATE() function in the SELECT list may look like:
select case
when ISDATE(DateString) = 1 then cast(DateString as datetime)
else null --or other error result
end as CastedDate
from TableName

Answer 2

If the logic is complicated (e.g. you want to try to correct the errors, such as missing slash, or nearest day in the case of 30th FEB), then one option is to create a user-defined function that contains the date parsing logic (string to date) logic in it, complete with error handling (e.g. scenario checking before casting). Then in the query, call the user defined function.
An outline:
CREATE FUNCTION udf_ParseDateString
(
#DateString nvarchar(20)
)
RETURNS DateTime
AS
BEGIN
DECLARE #returnDateTime datetime
-- Do any string checking, and date casting here
-- #DateString -> #returnDateTime
return #returnDateTime
END
Note that you won't be able to use TRY-CATCH in a UDF.
Alternatively, if your logic is simple, you could just use a cast inline, as suggested here.
SET DATEFORMAT ymd;
-- Incorporate this into query.
SELECT CASE WHEN ISDATE(#yourParameter) = 1
THEN CAST(#yourParameter AS DATETIME)
ELSE YourDefaultValue
END
If you were using SQL Server 2012, you could use the TRY_CAST function.