Oracle 18c

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with 
function startpoint_x(shape in sdo_geometry) return number 
is
begin
  return 
  shape.sdo_ordinates(1); 
end;
    
select
 startpoint_x(shape) as startpoint_x
from
  (select sdo_geometry('linestring(1 2, 3 4, 5 6)') as shape 
   from dual)


STARTPOINT_X


1


select
  (shape).sdo_ordinates(1) as startpoint_x
from
  (select sdo_geometry('linestring(1 2, 3 4, 5 6)') as shape 
   from dual)
ORA-00904: "MDSYS"."SDO_GEOMETRY"."SDO_ORDINATES": invalid identifier

select
  (shape).sdo_ordinates as ordinates 
  --db<>fiddle doesn't know how to display the result. It would look like this in SQL developer: SDO_ORDINATE_ARRAY(1, 2)
from
  (select sdo_geometry('linestring(1 2, 3 4, 5 6)') as shape 
   from dual)