Boolean expression in CASE in WHERE clause does not work - sql

Question

I am having problems with Firebird SQL statement in version Firebird 2.5.
Based on today's date, I have to select either this month's data or the previous month's data.
SELECT * FROM FA_DOBAVNICA
WHERE
1=1
AND CASE WHEN
extract(day from cast('Now' as date)) < 9
THEN
DATUM_NAROCILA BETWEEN 'start of previous month' AND 'end of previous month'
ELSE
DATUM_NAROCILA BETWEEN 'start of this month' AND 'end of this month'
END
I am getting a 104 error Token unknown for BETWEEN. I have no idea what am I doing wrong.

Answer 1

If I understood your problem correctly you can rephrase it with an or clause like this and it should do the job:
SELECT * FROM FA_DOBAVNICA
WHERE
1=1
AND
((extract(day from cast('Now' as date)) < 9 AND DATUM_NAROCILA BETWEEN 'start of previous month' AND 'end of previous month')
or
(extract(day from cast('Now' as date)) > 8 AND DATUM_NAROCILA BETWEEN 'start of this month' AND 'end of this month'))

Answer 2

Personally, I would avoid forking out conditions variants in where and make it two queries instead. I suspect conditional where might suppress Firebird's query optimizer, and two distinct queries connected together might end up with a better query plan using indexes.
Especially if you have more conditions than actually shown there which 1=1 placeholder implies. You have to check and compare real plans using you real data and real added conditions.
SELECT * FROM FA_DOBAVNICA
WHERE extract(day from cast('Now' as date)) < 9
AND DATUM_NAROCILA BETWEEN 'start of previous month' AND 'end of previous'
UNION ALL
SELECT * FROM FA_DOBAVNICA
WHERE extract(day from cast('Now' as date)) >= 9
AND DATUM_NAROCILA BETWEEN 'start of this month' AND 'end of this month'
However in your specific case - why using CASE at all? Why not computing the target date span instead?
WITH
TargetDay as
(SELECT DATEADD( -8 DAY TO CURRENT_DATE) AS TPoint FROM RDB$DATABASE)
,TargetStart as
(SELECT DATEADD( 1 - EXTRACT(DAY FROM TPoint) DAY TO TPoint) AS TStart FROM TargetDay)
,TargetEnd as
(SELECT DATEADD( -1 DAY TO DATEADD( 1 MONTH TO TStart)) AS TEnd FROM TargetStart)
select TStart, TEnd, TPoint from TargetStart, TargetEnd, TargetDay
TSTART
TEND
TPOINT
2021-08-01
2021-08-31
2021-08-03
db<>fiddle here
See? you do not need any if-then-else at all!
Added bonus is that you now can easily turn your 9 into an SQL parameter, rather than having it a literal constant injected into SQL code via always fragile string splicing, because now you only use 9 once in your query, thus you no more have have to care about changing two different parameters/constants in a query always in sync.
WITH
TargetDay as
(SELECT DATEADD( 1 - (cast( ? as integer )) DAY TO CURRENT_DATE) AS TPoint FROM RDB$DATABASE)
,TargetStart as....
Or in languages like Delphi which simulate named parameters for Firebird, it can be like
WITH
TargetDay as
(SELECT DATEADD( 1 - (cast( :ThresholdDay as integer )) DAY TO CURRENT_DATE) AS TPoint FROM RDB$DATABASE)
,TargetStart as....