Question

My dbase has date formats in 6 digits with a int datatype (‘734503’ = 01/01/2012).
I have been able to successfully convert these to char with:

SELECT CONVERT(char(12),dateadd(dd,(date_paid -    639906),'1/1/1753'),101)  
FROM   vouchers       
WHERE date_paid =    '734503'

This gives me the output of 01/01/2012.
But if I search on the converted date in a query like this:

SELECT CONVERT(char(12),dateadd(dd,(date_paid - 639906),'1/1/1753'),101) 
FROM vouchers    
WHERE date_paid >= '09/01/2012'  AND date_paid <= '09/30/2012'

Why doesn’t the conversion take place? Even if I use a CONVERT in the WHERE statement on the date_paid field, shouldn’t it work there?

I suppose my question is how do I search with the converted character date and not have to use the 6 digit date?

Answer 1

Your problem is that you are comparing an INT column with STRING values, and expecting it to resolve as if comparing dates…

WHERE date_paid >= '09/01/2012'
  AND date_paid <= '09/30/2012' 

-- date_paid is an INT (according to your question)
-- '09/01/2012' is a STRING

Because of the differing datatypes there is an implicit CAST() in there. Effectively you’re doing…

WHERE CAST(date_paid AS VARCHAR(10)) >= '09/01/2012'
  AND CAST(date_paid AS VARCHAR(10)) <= '09/30/2012' 

-- NOTE:  All of these values are now strings
--        They may LOOK like dates, but they're just strings

What you really need to do is explicitly manipulate the strings in to integers.

WHERE date_paid >= DATEDIFF(DAY, '01/01/1753', '09/01/2012') + 639906
  AND date_paid <= DATEDIFF(DAY, '01/01/1753', '09/30/2012') + 639906

This too involves implicit casts. DATEDIFF() only takes DATETIME datatypes so the strings are implicitly cast to DATETIMEs first.

EDIT:

Another option would be to CAST() the date_paid field into a DateTime, then base the WHERE caluse on that. The down side here is…
– The CAST() would have to be done on every row, then the WHERE clause applied
– This prevents any use of indexes and dramatically reduces performance

The answer above does all the manipulation on the constant values, so that the searched field can be processed in it’s native state; thus allowing use of indexes.