Question

I am trying to clean up the where clause statement in the following code:

SELECT
    CONVERT(datetime, [UTC_Time_Stamp], 127) AS TimeStamp
FROM 
    Table 
WHERE 
    CASE 
       WHEN ISDATE([UTC_Time_Stamp]) = 1 
       THEN CONVERT(datetime, [UTC_Time_Stamp], 127) 
       ELSE CAST('1/1/1900' AS datetime) 
    END > CAST('11/09/2012' AS datetime) 
    AND 
       CASE 
          WHEN ISDATE([UTC_Time_Stamp]) = 1 
          THEN CONVERT(datetime, [UTC_Time_Stamp], 127) 
          ELSE CAST('1/1/3000' AS datetime) 
       END < CAST('11/10/2012' as datetime) 
ORDER BY 
    TimeStamp;

UTC_Time_Stamp is stored as a string and is sometimes null. I was previously running into a conversion error inside my where clause. I fixed the error following advice from this question here, but I feel like there has to be a simpler way to achieve the same result.

Answer 1

You need to do the conversion within a case statement. SQL is a descriptive language and does not guarantee the order of processing. So, a where clause does not necessarily happen before other processing, even when it is in a subquery or a CTE. However, SQL does guarantee the order of processing in a case statement (that contains no expressions with aggregation functions).

You can simplify your statement by using a subquery:

select TimeStamp
FROM (select t.*,
             Case When ISDATE([UTC_Time_Stamp]) = 1 Then CONVERT(datetime, UTC_Time_Stamp, 127) end) as TimeStamp
      from Table t
     ) t
WHERE coalesce(TimeStamp, cast('1/1/1900' as datetime)) > cast('11/09/2012' as datetime) and
      coalesce(TimeStamp, cast('1/1/3000' as datetime)) < cast('11/10/2012' as datetime) 
ORDER BY TimeStamp;

This does the conversion to TimeStamp, for valid values. You can then use the variable in the outer query.

I would also encourage you to get used to the ANSI standard format for dates (YYYYMMDD or YYYY-MM-DD) instead of ambiguous formats like ’11/10/2012′. It may be clear to you that this means 2012-11-10 . . . or is that 2012-10-11 . . . but the format is ambiguous.