Query or Procedure?

Question

Hi all,
i try to explain my need by an example because i dont know how to tell it.
i have a table like this :
product | year  | quantity
paper   | 2008  | 147 
cd      | 2008  | 258 
paper   | 2009  | 369 
cd      | 2009  | 456 
end so on ... 2007 2006i would like to obtain a result like this
product | *current period* quantity | *previous period* quantity
paper   | 369                   | 147 
cd      | 456                   | 258  where period is not always the current year ( sysdate )
in order to be able to put it in a view.
i would like to know if exist a way to do something like:
select product, case year when ??year?? then quantity else 0, case year when ??year-1?? then quantity else 0using the value of field year as a kind of parameter.
if its not possible ( or even if it is ) its better to use a procedure?
Thanks to all.
Regards
Alessandro.

Answer 1

Hi, Allesandro,
The usual way to have parameterized views in Oracle is to store the parameters someplace where the view can reference them.
SYS_CONTEXT variables are often used for this, because several sesssions can each have their own values of the prameters, and run queries on the same view at the same time. SYS_CONTEXT can only store strings, so you often have to use conversion functions, like TO_NUMBER.
For example:
EXEC  set_view_x_year (2009);
SELECT    product
,       SUM ( CASE year
                     WHEN  TO_NUMBER (SYS_CONTEXT ('my_app', 'view_x_year'))
               THEN  quantity
               ELSE  0
          END
           )          AS current_quantity
,       SUM ( CASE year
                     WHEN  TO_NUMBER (SYS_CONTEXT ('my_app', 'view_x_year')) - 1
               THEN  quantity
               ELSE  0
          END
           )          AS previous_quantity
FROM      table_x
GROUP BY product
;To use a view like this, you first have to call a procedure (such as set_view_x_year, above) to set the SYS_CONTEXT varaible(s).
The varaibels will stay defined that way until you end the session (or you explicitly re-set them, of course), so you don;'t have to call the procedure every time you use the view.
You may want to build in default values, so the view will have some meaning even if you don't call the set-up procedure.
For example, to make the current year the default, you could replace
TO_NUMBER (SYS_CONTEXT ('my_app', 'view_x_year'))with
TO_NUMBER ( NVL ( SYS_CONTEXT ('my_app', 'view_x_year')
              , TO_CHAR (SYSDATE, 'YYYY')
       )     )