Compare date columns - teradata

Question

I need to retrieve the rows that service_date is greater than prod_date.
The data type for prod_date is VARCHAR(10) (2/20/2014 for example) and the data type for service_date is DATE (YYYYMMDD). If I query service_date using "select service_date from service where service_date ='20140201'", the result is showing "2/1/2014" in the result grid. However, it does not work in the query below when I convert service_date to varchar to compare with prod_date. It pulls out all the rows instead of the ones that have greater service_date.
SELECT P.PROD_ID, P.PROD_DESC, S.PROD_ID, S.SERVICE_LOC
FROM PRODUCT P
INNER JOIN SERVICE S
WHERE P.PROD_ID = S.PROD_ID
AND CAST(S.SERVICE_DATE AS VARCHAR(10)) >= P.PROD_DATE

Answer 1

I suggest you use date ordering instead of string/varchar ordering if possible for simplicity and since its [ probably ] closer to what your interested in and less likely to confuse
For example
'01/02/2014' >= '04/01/2013' -- if these are dates or cast to dates
but
'01/02/2014' < '04/01/2013' -- if these are strings
So to keep things simple, it makes sense to cast PROD_DATE to a date when comparing these two fields like :
SELECT P.PROD_ID, P.PROD_DESC, S.PROD_ID, S.SERVICE_LOC
FROM PRODUCT P
INNER JOIN SERVICE S
WHERE P.PROD_ID = S.PROD_ID
AND S.SERVICE_DATE >= cast(P.PROD_DATE as date format 'DD/MM/YYYY')
;
if theres any doubts on prod_dates quality as valid dates can check the conversion on all dates first ( before running/adjusting above )

Answer 2

This isn't 100% error proof given your date is character and could have unexpected values. It does show how you can append a leading 0 to the month value and cast it to a date when the month is determined to be a single digit based on the location of the / in the second position of the PROD_DATE value for a given row.
SELECT CASE WHEN POSITION('/' IN TRIM(P.PROD_DATE)) = 2
THEN CAST('0'|| TRIM(P.PROD_DATE) AS CHAR(10)) AS DATE FORMAT 'MM/DD/YYYY')
ELSE CAST(P.PROD_DATE AS DATE FORMAT 'MM/DD/YYYY')
END AS PROD_DATE_
FROM PRODUCT P;