What is the difference between "IS NOT NULL" and "NOT IN NULL"? Why these two Oracle queries return different results? - oracle

Question

SELECT DUMMY FROM DUAL
WHERE DUMMY IS NOT NULL;
Result: Returns 'X';
SELECT DUMMY FROM DUAL
WHERE DUMMY NOT IN NULL;
Result: Returns nothing.

Answer 1

In Oracle binary expressions can evaluate to three states: TRUE, FALSE and NULL where NULL can be thought of as the same as a undefined or unknown value. Applying a binary operation to a NULL value will give a NULL output (and not, as you might expect, TRUE or FALSE):
Boolean Operation Result
----------------- ------
X = X TRUE
X = Y FALSE
X = NULL NULL
NULL = X NULL
NULL = NULL NULL
NULL IN ( NULL ) NULL
NOT TRUE FALSE
NOT FALSE TRUE
NOT NULL NULL
The query:
SELECT DUMMY FROM DUAL WHERE DUMMY NOT IN ( NULL )
is the equivalent of:
SELECT DUMMY FROM DUAL WHERE NOT( DUMMY = NULL )
And the WHERE clause evaluates NOT( DUMMY = NULL ) to NOT NULL which in turn evaluates to NULL. Since the WHERE filter evaluates to a non-TRUE value the row is excluded.
The IS NULL operation is specifically designed to test whether a value is NULL and will do what you intend where the equality = operator or IN operator will not.
So:
Boolean Operation Result
----------------- ------
NULL = NULL NULL
NULL IN ( NULL ) NULL
NULL IS NULL TRUE

Answer 2

DUMMY in DUAL is a varchar2, and you selected all DUMMY from DUAL that are not null (IS NOT NULL). DUAL has one row with DUMMY='X'.
Now you are saying that DUMMY is not IN a set of NULL, which makes no sense really and the select will return no result.
NULL is a special character, and is not zero. It represents the absence of real data (data which includes zeroes and empty spaces).