How to use variables in Oracle PL/SQL Function - oracle

Question

I'm sorry upfront because this question seems to easy.
I have this function:
CREATE OR REPLACE FUNCTION Costs_MK (VIEWNAME IN VARCHAR2 , WHERE_CLAUSE IN VARCHAR2)
RETURN VARCHAR2
IS
v_Costs VARCHAR2 (500);
BEGIN
Select Listagg(Costs, ';' ) WITHIN GROUP (ORDER BY Costs)
into v_Costs
from (select distinct (Costs)
from VIEWNAME
where WHERE_CLAUSE);
RETURN v_Costs;
END Costs_MK;
However I get the Error-Message:
Error(13,30): PL/SQL: ORA-00920: invalid relational operator
I even can't compile it. If I use the exact values for Viewname and Where_clause I get the desired result.
What am I doing wrong?
/edit: Line 13 is
from VIEWNAME
/edit #2:
Thanks guys. You helped me a lot. I didn't thought about dynamic sql in the first step, so thanks for the refresher ;).

Answer 1

I suggest you to add EXCEPTION BLOCK along with EXECUTE IMMEDIATE
I have created a PROCEDURE you can similary create FUNCTION
CREATE OR REPLACE procedure Costs_PK(VIEWNAME IN VARCHAR2 , WHERE_CLAUSE IN VARCHAR2 )
AS
v_Costs VARCHAR2 (500);
sql_stmnt varchar2(2000);
BEGIN
sql_stmnt := 'Select Listagg(Cost, '';'' ) WITHIN GROUP (ORDER BY Cost) from (select distinct (Cost) from ' || VIEWNAME || ' where ' || WHERE_CLAUSE || ' ) ';
--sql_stmnt := 'Select Listagg(Cost, '';'' ) WITHIN GROUP (ORDER BY Cost) from (select distinct (Cost) from cost_tab where cost >=123 ) ';
EXECUTE IMMEDIATE sql_stmnt INTO v_Costs ;
dbms_output.put_line ('query works -- ' || v_costs);
EXCEPTION
WHEN OTHERS THEN
DBMS_OUTPUT.PUT_LINE ('input :' || VIEWNAME || ' and ' || WHERE_CLAUSE );
dbms_output.put_line (sql_stmnt );
DBMS_OUTPUT.PUT_LINE ('ERROR MESSAGE : ' || sqlCODE || ' ' || SQLERRM );
END;
begin
Costs_PK('cost_tab','cost >= 123');
end;
NOTE: code has been Tested
output:
query works -- 123;456

Answer 2

This is one of the areas in PL/SQL where the most straightforward static SQL solution requires code duplication as there is no way to parametrize the table name in a query. Personally I usually favor duplicate code of static SQL over the increased complexity of dynamic SQL as I like PL/SQL compiler to check my SQL compile time. YMMV.
You don't tell us what kind of where statements the different views are having. In the example below I assume there is 1:1 relation between the view and where parameter(s) so I can easily build static SQL.
create or replace view foo_v (foo_id, cost) as
select level, level*10 from dual connect by level < 10
;
create or replace view bar_v (bar_id, cost) as
select level, level*100 from dual connect by level < 10
;
create or replace function cost_mk(
p_view in varchar2
,p_foo_id in number default null
,p_bar_id in number default null
) return varchar2 is
v_cost varchar2(32767);
begin
case lower(p_view)
when 'foo_v' then
select listagg(cost, ';' ) within group (order by cost)
into v_cost
from (select distinct cost
from foo_v
where foo_id < p_foo_id);
when 'bar_v' then
select listagg(cost, ';' ) within group (order by cost)
into v_cost
from (select distinct cost
from bar_v
where bar_id < p_bar_id);
end case;
return v_cost;
end;
/
show errors
Usage example
select cost_mk(p_view => 'foo_v', p_foo_id => 5) from dual;
select cost_mk(p_view => 'bar_v', p_bar_id => 5) from dual;

Answer 3

You would want to use EXECUTE IMMEDIATE as was hinted by the comments to your question, define a new variable sqlQuery VARCHAR2(200); or similar and rework your sql similar to the following:
sqlQuery := 'Select Listagg(Costs, '';'' ) WITHIN GROUP (ORDER BY Costs) ';
sqlQuery := sqlQuery || 'from (select distinct (Costs) from :1 where :2)';
EXECUTE IMMEDIATE sqlQuery INTO v_Costs USING VIEWNAME, WHERE_CLAUSE;