Convert varchar to date with malformed values in SQL Server 2008 - sql-server

Question

This whole date code is beyond my understanding but non the less I need to convert a column in a query result from varchar to date. I have some malformed values and I'm trying to filter them out.
SET DATEFORMAT dmy
SET LANGUAGE us_english
select convert(datetime, some_date, 103)
from some_table
where isdate(some_date) = 1
I keep getting
Conversion failed when converting date and/or time from character string
When querying for the malformed values I get these two records
some_date
---------
621
232
Is there any way to validate the format using isdate ?
Thanks, Frustrated Oracle DBA.
EDIT
Tried #Sean Lange suggestions and indeed it looked promising
select convert(datetime, some_date, 103)
from ( select * from some_table where isdate(some_date) = 1 )
But then I added a where clause
select convert(datetime, some_date, 103)
from ( select * from some_table where isdate(some_date) = 1 )
where convert(datetime, some_date ,103) between
convert(datetime, '01/07/2014')
and convert(datetime,'31/07/2014')
How does a where clause being executed before the from?

Answer 1

Since we know that a valid date looks like dd/mm/yyyy how about a simple structure test?
select convert(datetime, some_date, 103)
from some_table
where len(some_date) = 10 --must have exactly the expected number of characters
This could be expanded upon (depending on requirements)
and some_date like '__/__/____'
Or maybe
and some_date like '[0-9][0-9]/[0-9][0-9]/[0-9][0-9][0-9][0-9]'
Anything better would probably need you to put together a custom function that does the CONVERT in a try block, returning NULL in the catch block if its an invalid record. Depends on your requirements...

Answer 2

The problem is happening and will continue to happen because you can't be certain which order the sql engine will take here. One time it might filter the rows first and another time it does the conversion. So what you need to do is force the order the engine works. We can do this by using a CTE to get only the rows we want to convert.
with MyCTE as
(
select some_date
from some_table
where ISDATE(some_date) = 1
)
select CONVERT(datetime, some_date, 103)
from MyCTE

Answer 3

If you're positive that these are the only malformed values and all malformed values will look like this, you could just go ahead and use a hackish CASE statement
SELECT (CASE WHEN LEN(datetime) > 3 THEN convert(datetime, some_date, 103) ELSE NULL),
...
It's really not elegant, but datetimes are notoriously difficult to make elegant because there are so many fragmented formats. If possible, it saves more headaches just to make sure that your data is correctly formatted in the first place through form validation and regex stuff, but since that's not always possible, we have crummy hacks like this to patch ourselves through. Best of luck!