Please help with the sql logic

Question

The below is going to be a record set from for a view from the FROM CLAUSE
1. Find component DOCUMENT where the document type = Square Metres (DOC.UOM_STD_ID = 'METR_MTR) If any components are found, retrieve the Document records for these components ONLY.
2. Otherwise, if no components exist with a Document type = Square Metres, look for components with a Document type of Acres (DOC.UOM_STD_ID = 'ACRE'). If found, retrieve the Space Lease records for these components ONLY.
3. Otherwise, if the Document does not have components with a type = Square Metres or Acres, look for components with the type of (DOC.UOM_STD_ID = 'PARK'). Then retrieve the departments associated with these parking components.
For Space Lease records above, retrieve the associated Org ID (SPACE.SPORG).
Return all departments where
the associated Org Type (ORG.ORGCLASS) = 'NON'
or the associated Org Type (ORG.ORGCLASS) = 'GD'
Thank you

Answer 1

maybe something like:
SQL> select emp.empno, emp.ename, emp.deptno
  2    from emp,
  3         (select case when (select 1 from emp where deptno = 10 and rownum  = 1) = 1 then 1
  4                      when (select 2 from emp where deptno = 60 and rownum  = 1) = 2 then 2
  5                      else 3
  6                 end case_col
  7            from dual) d1
  8   where deptno = decode(d1.case_col,1,10,
  9                                     2,60,
10                                     3,70);
     EMPNO ENAME          DEPTNO
      7839 KING               10
      7782 CLARK              10
      7934 MILLER             10
SQL> in you code it will be something like:
select ...
   from document t1,        
        (select case when (select 1 from document doc where DOC.UOM_STD_ID = 'METR_MTR' and rownum  = 1) = 1 then 1
                     when (select 1 from document doc where DOC.UOM_STD_ID = 'ACRE' and rownum  = 1) = 2 then 2
                     else 3
                end case_col
           from dual) d1
where t1.uom_std_id = decode(d1.case_col,1,'METR_MTR',
                                         2,'ACRE',
                                         3,'PARK',
                                           null);note: untested