Question

I am querying tables from Microsoft SQL 2008 which have date split across 3 columns: day, month and year. Unfortunately, I do not have control over this because data is coming in to the database daily from a 3rd party source in that format.

I need to add between to a where clause so user can pull records within a range. Would be easy enough if date was in a single column but finding it nearly impossible when its split across three columns.

To display the date, I am doing a

CAST(
    CAST(year as varchar(4)) + '-' + 
    CAST(month as varchar(2)) + '-' + 
    CAST(day as varchar(2))
as date) AS "date"`  

in a select. I tried to put it as a parameter for datediff function or just the regular between but get no results.

Answer 1

Without access to change the table, you can use a subquery to construct the date before the filtration is applied:

SELECT x.*
  FROM (SELECT CAST(CAST(year as varchar(4)) + '-' + 
                    CAST(month as varchar(2)) + '-' + 
                    CAST(day as varchar(2)) AS date) AS [date]
                    ...
          FROM ...) x
 WHERE x.[date] BETWEEN ? AND ?

If using SQL Server 2005+, you can use a Common Table Expression (CTE) instead — but there’s no performance difference between using a CTE vs the subquery approach:

WITH example AS (
   SELECT CAST(CAST(year as varchar(4)) + '-' + 
               CAST(month as varchar(2)) + '-' + 
               CAST(day as varchar(2)) AS date) AS [date]
               ...
     FROM ...) 
SELECT x.*
  FROM example x
 WHERE x.[date] BETWEEN ? AND ?

Caveats:

Because the dates are being constructed on the fly, this will never perform as well as if the values were stored as DATE or DATETIME — an index on the year, month or day columns can not be utilized. Options to consider would be an indexed view (AKA materialized view), or computed column.