This will help not only other people, but you too when debugging the query. Do you really believe you'll remember all column names in INSERT statements next year? Some of them you might, and some you won't. Also, what if the table descrption changes? If you add another NOT NULL column? You'll be in trouble.
Now, about your query: UPDATE you wrote is kind of lucky as it is a good candidate to end up with the TOO-MANY-ROWS error in line 'supervisorID = (SELECT empID ...)'. Better choice would be 'IN' instead of '='. Therefore, such a query might help you:
UPDATE employee SET
salary = 50000
WHERE empID IN (SELECT supervisorID FROM employee);
This will give 50000 to every employee whose ID is among supervisorID's in the table.