oracle select from table if values not exists

You can enclose the SELECT statement within a block and add an exception handler to it.

So, if there are no rows selected from the first table, then you select from the second table. The structure would be something like below:

Begin
    select <many columns> into <many variables or row type>
    From Table1
    where <conditions>;
EXCEPTION
    WHEN NO_DATA_FOUND THEN

    select <many columns> into <many variables or row type>
    From Table2
    Where <Conditions>;
End;

References:

Another related SO question

Exception Handlers

Documentation for the SELECT INTO statement

Here is an example of a PL/SQL function that will perform a test, and then execute a secondary query based upon the results of the test. You can adjust it to fit your needs:

set serveroutput on;

declare
  row_count number;
  column1 varchar(10);
  column2 varchar(10);
  column3 number;

begin

  /*Perform your test*/
  select count(target_column) into row_count
  from my_table
  where condition_column = 'x';

  /*Run your secondary query based on the output of the first*/
  if row_count > 0 then
    select 
      col_x into column1,
      col_y into column2,
      col_z into column3
    from my_other_table_A;
  else
    select 
      col_a into column1,
      col_b into column2,
      col_c into column3
    from my_other_table_B;
  end if;

  /*Show the results*/
  dbms_output.put_line('column1: ' || column1);
  dbms_output.put_line('column2: ' || column2);
  dbms_output.put_line('column3: ' || column3);

end;
/