SQL>
SQL>   CREATE TABLE "TEST_TAB"
  2     (       "KEY1" NUMBER(10));

Table created.

SQL>
SQL>    insert into "TEST_TAB" values ('1');

1 row created.

SQL>    insert into "TEST_TAB" values ('2');

1 row created.

SQL>    insert into "TEST_TAB" values ('3');

1 row created.

SQL>    insert into "TEST_TAB" values ('4');

1 row created.

SQL>    commit;

Commit complete.

SQL>
SQL>

SQL> create or replace function is_num_in_list (p_number  number)
  2                                             return number
  3  is
  4  TYPE varray_type IS TABLE OF NUMBER(10) INDEX BY PLS_INTEGER;
  5  v_my_array varray_type;
  6  begin
  7  select key1 into v_my_array from TEST_TAB;
  8    --if the number provided exists in the list, then 1(TRUE)
  9  if p_number in v_my_array then
 10  return(1);
 11  else
 12      --if the number provided *IS NOT* in the list, then 0(FALSE)
 13    return(0);
 14  end if;
 15  end;
 16  /

Warning: Function created with compilation errors.

SQL> show error
Errors for FUNCTION IS_NUM_IN_LIST:

LINE/COL ERROR
-------- -----------------------------------------------------------------
9/16     PLS-00103: Encountered the symbol "V_MY_ARRAY" when expecting one
         of the following:
         (

SQL>

SQL> CREATE TABLE "TEST_TAB"
  2    2     (       "KEY1" NUMBER(10));
  2     (       "KEY1" NUMBER(10))
  *
ERROR at line 2:
ORA-00922: missing or invalid option

 insert into "TEST_TAB" values ('1');

7  select key1 BULK COLLECT into v_my_array from TEST_TAB;

if p_number MEMBER OF v_my_array then

SQL> select * from TEST_TAB order by 1;
      KEY1
----------
        10
        20
        30
        40

4 rows selected.

SQL> create or replace function is_num_in_list (p_number  number)
  2   return number
  3  is
  4  TYPE varray_type IS TABLE OF NUMBER(10);
  5  v_my_array varray_type;
  6  begin
  7    select key1 bulk collect into v_my_array from TEST_TAB;
  8    if p_number member of v_my_array then
  9      return(1);
 10    else return (0);
 11    end if;
 12  end;
 13  /

Function created.

SQL> select is_num_in_list (0) from dual;
IS_NUM_IN_LIST(0)
-----------------
                0

1 row selected.

SQL> select is_num_in_list (10)  from dual;
IS_NUM_IN_LIST(10)
------------------
                 1

1 row selected.

SQL> create or replace function is_num_in_list (p_number  number)
  2   return number
  3  is
  4   v pls_integer;
  5  begin 
  6    select 1 into v from TEST_TAB where key1 = p_number and rownum = 1;
  7    return 1;
  8  exception
  9    when no_data_found then return 0;
 10  end;
 11  /

Function created.

SQL> select is_num_in_list (0) from dual;
IS_NUM_IN_LIST(0)
-----------------
                0

1 row selected.

SQL> select is_num_in_list (10)  from dual;
IS_NUM_IN_LIST(10)
------------------
                 1

1 row selected.