Using CASE within a where clause - with a parameter - sql

Question

I am looking for a good example of a case statement inside a where clause, using a variable. The number returned by the variable determines the result of the where clause. My query looks like this:
IF #RequestType = 99
SELECT #Received = count(*)
FROM Request r
WHERE request_received_date between #beginDate and #endDate
AND request_type_id <> 5
ELSE IF #RequestType = 100
BEGIN
SELECT #Received = count(*)
FROM Request r
WHERE request_received_date between #beginDate and #endDate
END
ELSE
BEGIN
SELECT #Received = count(*)
FROM Request r
WHERE request_received_date between #beginDate and #endDate
AND request_type_id =
(CASE WHEN #RequestType = 1 THEN (1)
WHEN #RequestType = 2 THEN (2)
WHEN #RequestType = 3 THEN (3)
WHEN #RequestType = 4 THEN (4)
WHEN #RequestType = 5 THEN (5)
END)
END
So as you can see, there are 7 options - the customer wants to be flexible with the amount of data they see in the report. This looks cumbersome to me but I can't think of any other way to do this.

Answer 1

you could probably simplify the query to...
SELECT
count(*) as Received
FROM
( select 3 as RqType,
'2015-10-01' as beginDate,
'2015-10-20' as endDate) as PV,
Request r
WHERE
request_received_date between PV.beginDate and PV.endDate
AND case when PV.RqType = 100 then 1=1
when PV.RqType = 99 then r.request_type_id <> 5
when PV.RqType <= 6 then PV.RqType = r.request_type_id
end
The LAST case/when component matches on the request type is getting the values under 6 to match that of the request type. If you have negatives, then just adjust that accordingly.
I revised the query to have a prequery "PV" indicating the parameter values you are looking for, so they can just be changed to whatever request type and date-range as you need. This should be good for SQL-Server, and probably other SQL engines too. By just doing a query of hard values with no "FROM", it essentially creates a result set of a single record with those values of which you can then use for the rest of the query. Any by being a single record, no issue for any Cartesian joins.

Answer 2

There is usually no need for CASE WHEN in WHERE clauses, as you have AND and OR. Your queries translate to:
select #Received = count(*)
from request
where request_received_date between #beginDate and #endDate
and
(
#RequestType = 100
or
(#RequestType = 99 and request_type_id <> 5)
or
(#RequestType = request_type_id and request_type_id in (1,2,3,4,5))
);