Start a Case Statement without an outside 'Select' - CTE's - sql

Question

Running this query (below) returns a 'Too Many Values' error:
select
case
when to_char(sysdate, 'yyyymmdd') = to_char(sysdate, 'yyyymm') || '01'
then (select FirstReportGroups.*, FirstReportDetails.*
from FirstReportGroups, FirstReportDetails)
when to_char(sysdate, 'yyyymmdd') = to_char(sysdate, 'yyyymm') || '16'
then (select SecondReportGroups.*, SecondReportDetails.*
from SecondReportGroups, SecondReportDetails)
end as LetsSee
from cmtmpentered t1 join cmtmpconf t2
on t1.casenumber = t2.casenumber
and t1.enc = t2.enc
;
I'm using CTE's (they are not included because it would make this very long) and it makes logical sense to me, but googling this 'Too Many Values' error gives me no substantial help. Running the CTE's individually works, so that is not the problem.
I think all would be solved if I could only get rid of the outside 'Select' statement and just keep the selects inside the Case. If I'm explaining this poorly, an example of what I'm looking for is this:
case
when to_char(sysdate, 'yyyymmdd') = to_char(sysdate, 'yyyymm') || '01'
then (select FirstReportGroups.*, FirstReportDetails.*
from FirstReportGroups, FirstReportDetails)
when to_char(sysdate, 'yyyymmdd') = to_char(sysdate, 'yyyymm') || '16'
then (select SecondReportGroups.*, SecondReportDetails.*
from SecondReportGroups, SecondReportDetails)
end as LetsSee
;
Is this doable in any capacity? This syntax obviously doesn't work, otherwise I wouldn't be asking the question - but is there a related way I can do this?

Answer 1

select FirstReportGroups.*, FirstReportDetails.*
from (select 1 a from dual) FirstReportGroups
cross join (select 2 b from dual) FirstReportDetails
where extract(day from sysdate) = 1
---------
union all
---------
select SecondReportGroups.*, SecondReportDetails.*
from (select 3 a from dual) SecondReportGroups
cross join (select 4 b from dual) SecondReportDetails
where extract(day from sysdate) = 16;
Replaced common table expressions with inline views. CTEs should only be used if they are referenced more than once. They may look a little nicer with small examples and for programmers only used to procedural code. Serious SQL requires multiple nested levels of inline views. And debugging is much easier without CTEs - the CTEs make it difficult to highlight and run sub-blocks of code.
Replaced case expression with predicate to filter by date. A CASE expression can only return a single value. You could probably do something fancy with types, but that would be horribly complicated. My code still makes the assumption that the two sets return the same types of values. If that's not true you'll need to do something different at the application level.
Replaced to_char with extract. Date handling can be confusing in Oracle. The trick is to keep everything in their native type. You should almost never need to use to_char or to_date for anything other than formatting for display. For any other operation there's almost certainly a function to handle it without converting types.
Replaced , with ANSI-syntax cross join. The ANSI-style syntax has several advantages. One major advantage is it makes it clear when cross joins are intentional.

Answer 2

You aren't specifying a join condition for LetsSee. You are effectively cross joining LetsSee with the result of your from clause:
from cmtmpentered t1
join cmtmpconf t2 on t1.casenumber = t2.casenumber and t1.enc = t2.enc
I suggest you join LetsSee with t1 or t2.
Also, you are not specifying a join condition for the pairs:
FirstReportGroups and FirstReportDetails
SecondReportGroups and SecondReportDetails