CASE in WHERE Clause - sql

Question

Below is my current SQL Server 2012 query. Basically I want the information from the last business day, but on Monday, I want it to pull Friday's info instead of Sunday. This is what I have so far in my query but it won't accept it.
USE [LetterGeneration]
SELECT
g.LetterGenerationPrintJobId,
CONVERT(CHAR(12), r.CreatedDate, 101) AS CreatedDate,
YEAR(r.CreatedDate) AS Year,
MONTH(r.CreatedDate) AS Month,
DAY(r.CreatedDate) AS Day,
CASE
WHEN DATEPART(dw, r.CreatedDate) = 1
THEN 1
WHEN DATEPART(dw, r.CreatedDate) = 7
THEN 1
ElSE 0
END AS Weekend,
s.LetterGenerationStatusId AS Status,
COUNT(g.LetterGenerationId) AS LetterCount,
SUM(g.LetterPageCount) AS PageCount,
t.IsLitigationCoverLetterAllowed,
CASE
WHEN g.CarrierTrackingNumber LIKE '%1ZE%'
THEN 1
WHEN g.CarrierTrackingNumber LIKE '921489%'
THEN 2
WHEN g.CarrierTrackingNumber LIKE '917190%'
THEN 2
ELSE 3
END AS CarrierType
FROM
[LetterGenerationTemplateRequest] AS R
INNER JOIN
[LetterGenerationTemplate] AS T ON t.[LetterGenerationTemplateId] = r.LetterGenerationTemplateId
INNER JOIN
LetterGeneration G ON g.LetterGenerationTemplateRequestId = r.LetterGenerationTemplateRequestId
INNER JOIN
LetterGenerationStatus S ON g.LetterGenerationStatusId = s.LetterGenerationStatusId
WHERE
(CASE
WHEN (DATENAME(dw,GETDATE()) = 'Monday')
THEN (DATEDIFF(d, r.CreatedDate, GETDATE()) = 3)
ELSE (DATEDIFF(d, r.CreatedDate, GETDATE()) = 1)
END)
AND t.[TemplateKey] NOT LIKE '%PLTV1%'
AND s.LetterGenerationStatusId = 19
ORDER BY
r.CreatedDate DESC, g.LetterGenerationPrintJobId DESC
What am I missing or misunderstanding about my WHERE clause in order to make it work in the way I'm thinking?
Thanks

Answer 1

Maybe convert to a regular AND/OR?
WHERE (
((DATENAME(dw,GETDATE()) = 'Monday') AND (DATEDIFF(d, r.CreatedDate, GETDATE()) = 3))
OR
(DATEDIFF(d, r.CreatedDate, GETDATE()) = 1)
)
....

Answer 2

What am I missing or misunderstanding about my WHERE clause in order to make it work in the way I'm thinking?
Though you haven't given the error message you're getting, I'm sure it's syntax related because you're putting the test INSIDE the result of the case, not outside it
You're writing:
WHERE CASE WHEN it_is_monday THEN data_date = friday ELSE data_date = yesterday END
You should be writing:
WHERE data_date = CASE WHEN it_is_monday THEN friday ELSE yesterday END
Essentially: you're not supposed to use case/when in a where clause to do your "column = something" comparison and return you true or false, you're supposed to use it to just return the "something" you compare against "column" else in order to get your true or false
The other answers focus on "giving you a working solution"; this answer focuses on telling you what was going wrong with your thought processes re your original query
Here's a simpler example:
--wrong syntax to search a table full of cats (4 legs) and people (2 legs)
WHERE CASE WHEN animal_type = 'cat' THEN legs = 4 ELSE legs = 2 END
--right syntax
WHERE limbs = CASE WHEN animal_type = 'cat' THEN 4 ELSE 2 END

Answer 3

Ignoring holidays for a second, and assuming you have at least one record for every date, something like this should work.
where cast(createdDate as date) =
(select max(createdDate )
from table
where createdDate < cast(getDate() as date
and dateName(dw, createdDate in ('Monday' etc)
)

Answer 4

In order to maintain SARGability(able to do a seek against an index) you want to make sure the table columns in the predicate aren't included in any functions.
The following should work and maintain SARGability...
WHERE
r.CreatedDate = CASE
WHEN DATEPART(dw, getdate) = 2
THEN DATEADD(dd, -3, CAST(GETDATE() AS DATE))
ELSE CAST(GETDATE() AS DATE)
END
HTH,
Jason