Try it.
You will get different results...
SELECT * FROM MyTable WHERE Manager != NULL
Will give you no rows.
SELECT * FROM MyTable WHERE Manager IS NOT NULL
Will give you rows where there is a manager.
Why? Because according to
W3Schools[
^]
"It is not possible to test for NULL values with comparison operators, such as =, <, or <>."
NULLs propagate through expressions - anything involving a NULL value results in NULL. So when you use a condition involving an equality operator and one of the sides is NULL, the result is not TRUE or FALSE, it's NULL.
In fact, both of these will produce the same results:
SELECT * FROM MyTable WHERE Manager != NULL
SELECT * FROM MyTable WHERE Manager = NULL
Both give you no rows!