Oracle NVL returning inconsistence result when used with/without CASE statement

Can anybody help me to understand why oracle db is returning inconsistence result when used with/without CASE statement.

SELECT NVL(CASE WHEN '' IS NULL THEN NULL ELSE NULL END ,TO_DATE('19010101', 'YYYYMMDD')) a,
   NVL(CASE WHEN '' IS NOT NULL THEN NULL ELSE NULL END ,TO_DATE('19010101', 'YYYYMMDD')) b,
   NVL(CASE WHEN '' = '' THEN NULL ELSE NULL END ,TO_DATE('19010101', 'YYYYMMDD')) c,
   NVL(NULL ,TO_DATE('19010101', 'YYYYMMDD')) d from dual;

My actual query uses TRUNC(NVL(:NEW_FORM_TIME_END, TO_DATE('19010101', 'YYYYMMDD'))) >= TRUNC(SYSDATE - INTERVAL '2' DAY) where NEW_FORM_TIME_END could be empty string or NULL but I'm getting this error ORA-06550: line 240, column 91: PLS-00306: wrong number or types of arguments in call to '>=' ORA-06550: line 229, column 9: PL/SQL: Statement ignored

Your case expressions are of type VARCHAR2 which is the default type if you're not providing explicit types. A NULL literal doesn't have a type. NVL(<varchar2>, <date>) applies an implicit type conversion on the <date> argument. In other RDBMS, you'd simply get an error because of incompatible types. The PL/SQL version of your expression probably also suffers from a similar problem, although not exactly the same one as you're presenting in your query.

Just make sure you always compare the same types.

Try this to check:

SELECT 
  NVL(CAST(CASE WHEN '' IS NULL THEN NULL END AS DATE), DATE '1901-01-01') a,
  NVL(CAST(CASE WHEN '' IS NOT NULL THEN NULL END AS DATE), DATE '1901-01-01') b,
  NVL(CAST(CASE WHEN '' = '' THEN NULL END AS DATE), DATE '1901-01-01') c,
  NVL(NULL, DATE '1901-01-01') d
FROM dual;

Or alternatively, create a view from your table and then check the dictionary:

CREATE VIEW v AS
SELECT 
  NVL(CASE WHEN '' IS NULL THEN NULL END, DATE '1901-01-01') a,
  NVL(CASE WHEN '' IS NOT NULL THEN NULL END, DATE '1901-01-01') b,
  NVL(CASE WHEN '' = '' THEN NULL END, DATE '1901-01-01') c,
  NVL(NULL, TO_DATE('19010101', 'YYYYMMDD')) d
FROM dual;

SELECT column_name, data_type
FROM all_tab_cols
WHERE table_name = 'V'
ORDER BY column_name;

Yielding

|COLUMN_NAME|DATA_TYPE|
|-----------|---------|
|A          |VARCHAR2 |
|B          |VARCHAR2 |
|C          |VARCHAR2 |
|D          |DATE     |