CONVERT on only NOT NULL records - sql-server

Question

I am working in SQL Server 2008. I have a table with two columns that store date values, but the columns themselves are varchar. (I know, this is bad practice, but I don't own the table.) Some of the records in these two columns are NULL. I am trying to do a comparison of the two columns against each other. So, to enable the comparison, I need to do a CONVERT. But, CONVERT fails when it encounters NULLs. How do I essentially prevent the CONVERT from happening when it encounters a NULL? My core query is as follows:
SELECT
col1
FROM table_a
WHERE
CONVERT(date, col2, 101) > CONVERT(date, col3, 101)
I tried an inner query with an IN clause (i.e., the inner query returns only non-NULL records), but this failed because it seems that the query optimizer runs both queries independently, i.e., it runs the CONVERT on all records, which causes the failure. I also tried a self join (i.e., return only records in the first instance of the table where the records aren't null in the second instance of the table). But, this failed as well because of the same problem in the inner query scenario.

Answer 1

Try this:
SELECT
col1
FROM table_a
WHERE
case
when col2 is null or col3 is null then 0
when CONVERT(date, col2, 101) > CONVERT(date, col3, 101) then 1
end = 1
This method prevents the null strings from being converted (or attempted to be) and lets you decide what else you might want to do if one or the other cols is null.
EDITS: first version contained syntax errors in the case statement - these should be fixed now. Apologies for the hasty typing and not SQLFiddling it.

Answer 2

Try this: (Sample Data Included)
DECLARE #TABLE TABLE
(
SomeText VARCHAR(50),
Date01 VARCHAR(50),
Date02 VARCHAR(50)
)
INSERT INTO #TABLE VALUES('Good Date 01','10/01/2014','12/30/2014')
INSERT INTO #TABLE VALUES('Early Date 01','10/01/2014','12/30/2013')
INSERT INTO #TABLE VALUES('Good Date 02','10/01/2013','10/01/2014')
INSERT INTO #TABLE VALUES('Bad Data 01',NULL,'12/30/2014')
INSERT INTO #TABLE VALUES('Bad Data 02','10/01/2014',NULL)
INSERT INTO #TABLE VALUES('Bad Data 03',NULL,NULL)
SELECT
SomeText,
DATEDIFF(D,CONVERT (DATE, Date01, 101) , CONVERT (DATE, Date02, 101)) AS DELTA
FROM
#TABLE
WHERE
1 = 1
AND ISDATE(Date01) = 1
AND ISDATE(Date02) = 1
and DATEDIFF(D,CONVERT (DATE, Date01, 101) , CONVERT (DATE, Date02, 101)) < 0
The ISDATE(Date01) and ISDATE(Date02) remove missing data points from being attempted by the convert function.

Answer 3

Try
SELECT * FROM #Tmp
WHERE
col2 IS NULL OR col3 IS NULL OR
CONVERT(DATETIME, col2, 101) > CONVERT(DATETIME, col3, 101)
I have tried below query in SQL 2005 its working fine without any error, but it returns all the records without null. Above query return null record too
SELECT * FROM #Tmp WHERE CONVERT(DATETIME, col2, 101) > CONVERT(DATETIME, col3, 101)