When using variables in statements (see Variables syntax in SQL) for example like

SELECT * FROM users WHERE name = ##name::string::null;

, database engines will generally not handle correctly the case where a variable is valued to NULL. For instance this example would result in the code being executed as

SELECT * FROM users WHERE name = NULL;

which would probably give no result, whereas the expected equivalent code would be

SELECT * FROM users WHERE name IS NULL;

Libgda automatically handles theses cases and makes sure the IS NULL or IS NOT NULL constructions are used when necessary. It won't of course handle variable's NULL values when using other operators that equal or different, tough.