Update columns with null values

Hi I want to update a table as followings

update userloginstats set logouttime= sysdate where logouttime= null;

But it did not update the columns with null values. what is wrong

Change it to

...where logouttime is null;
                    ^^^^^^^

NULL is a special value and we cannot use the usual = operator with it.

From the Oracle documentation for NULL:

To test for nulls, use only the comparison conditions IS NULL and IS NOT NULL. If you use any other condition with nulls and the result depends on the value of the null, then the result is UNKNOWN because null represents a lack of data, a null cannot be equal or unequal to any value or to another null

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