Hibernate User Guide relevant section: https://docs.jboss.org/hibernate/orm/6.6/userguide/html_single/Hibernate_User_Guide.html#_nullif

HQL example:

select nullif(p.nickName, p.name) from Person p 

The above select field will return NULL if nickName is equal to name; otherwise will return nickName.

The NULLIF function can be represented in MQL using $cond.