A NULL comparison expression tests whether a single-valued path expression or an input parameter has a NULL value. Usually, the NULL comparison expression is used to test whether or not a single-valued relationship has been set.
SELECT t FROM Team t WHERE t.league IS NULL
This query selects all teams where the league relationship is not set. Please note, the following query is not equivalent:
SELECT t FROM Team t WHERE t.league = NULL
The comparison with NULL using the equals operator (=) always returns an unknown value, even if the relationship is not set. The second query will always return an empty result.