SQL WHERE clause

Question

I want to use dynamic where clause in select statement. For example in a query like
SELECT * FROM emp &where_clause_options
I am constructing my where_clause_option within a IF-ELSE-ENDIF construct which is soemthing like this.
if x=10 then
where_clause_option := 'where dept = 10 and job = 'CLERK'
else
where_clause_option := 'where sal >3000'
end if;
Now how to specify it in the select statement?
By the way DECODE statement does not work in WHERE clause.

Answer 1

Thanks for your suggestion. I know that decode does not work that way in where clause although it may be used for establishing list of columns in the SELECT startement. And that is exactly what has raised this issue that then how to get the dynamical where clauses from within the code. By the way my code is plsql based and I am running it in sqlplus environment.
I have tried to use the lexical parameter along with a combinition of DEFINE but define is uncontrolable in a conditional construct and hence the DEFINE in the last part of the IF then elsif else construct overrides previous defines and thus always passes to &my_where_clause1 the value defined in the last DEFINE statement. Here is how I tried to use Define:
DECLARE --main
v_sex varchar2(5);
my_where_clause1 varchar2(50);
Function myfunction return boolean is
begin
some code..........
if v_sex = 'MALE' then
DEFINE my_where_clause1 = 'and AGE > 60';
else
DEFINE my_where_clause1 = 'and AGE between 45 and 55';
end if;
some code......
end -- Function myfunction;
CURSOR mycursor is select patient_id from patient_history where Symptom = 'RESPIRATORY' &my_where_clause1;
BEGIN -- main
some code.....
if myfunction = TRUE then
for v_mycursor in mycursor Loop
some code.....
end loop;
end if;
END -- main;
Although this provides me with lexical parameter functionality within my PLSQL code but I am unable to control the DEFINE. Neither DEFINE-lexicial combinition nor the DECODE are helping in this scenario.
I am sure that there would be some way to implement it. So my call for help is still on.