Update columns with Null values
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
You cannot compare NULLs with =.
Use this:
update userloginstats set logouttime= sysdate where logouttime is null;
logouttime is null, not = null. null is never equal to anything, not even itself. Thus, the operator is.