You must be careful when searching a table that might contain nulls. For example, if the EdLev column contains nulls and you submit the following query, the result contains only the names of employees with an education level of less than 16 years.
SELECT Name, EdLev FROM Employee WHERE (EdLev < 16) ;
To ensure that the result of a statement contains nulls, you must structure it as follows.
SELECT Name, EdLev FROM Employee WHERE (EdLev < 16) OR (EdLev IS NULL) ;