Oracle case statement with is null

Question

I want something like this

IF EMP_ID_NULL = 'N' THEN
SELECT * FROM EMPLOYEE WHERE 
 EMP_ID IS NULL;
ELSE 
SELECT * FROM EMPLOYEE WHERE 
 EMP_ID =EMP_ID_VAL;
END IF;

And i tried the above in CASE statement like below, but as expected it dint work. How do i add is null in CASE?

DECLARE EMP_ID_NULL VARCHAR2(1);
DECLARE EMP_ID_VAL NUMBER;
SELECT * FROM EMPLOYEE WHERE 
 AND EMP_ID  ( CASE WHEN EMP_ID_NULL = 'N' THEN 'IS ' || NULL
                      ELSE '='|| EMP_ID_VAL END
                );

Answer 1

One way would be:

(NOTE: did include those variables using a WITH statement, because it is unknow to me how to create those on DBFIDDLE...)

WITH tmp_value as (
  SELECT 
     'N' as emp_id_null,
     2 as emp_id_val
)
SELECT 
  emp_id,
  emp_name,
  emp_id_null
FROM
  employee
CROSS APPLY tmp_value
WHERE EMP_ID_NULL ='N' and emp_id is null

UNION ALL

  SELECT 
  emp_id,
  emp_name,
  emp_id_null
FROM
  employee
CROSS APPLY tmp_value
WHERE EMP_ID_NULL !='N' and emp_id = emp_id_val
;

see: DBFIDDLE

EDIT: Or this (which I should have produces before doing above...)

WITH tmp_value as (
  SELECT 
     'Y' as emp_id_null,
     2 as emp_id_val
)
SELECT 
  emp_id,
  emp_name,
  emp_id_null
FROM
  employee
CROSS APPLY tmp_value
WHERE (EMP_ID_NULL ='N' and emp_id is null)
   or (EMP_ID_NULL !='N' and emp_id = emp_id_val);