Hibernate User Guide relevant section: https://docs.jboss.org/hibernate/stable/orm/userguide/html_single/Hibernate_User_Guide.html#hql-case-expressions

HQL examples:

Flavour #1: simple case expressions

select 
    case p.nickName 
	when 'NA' then '<no nick name>' 
        else p.nickName end
from Person p

Flavour #2: searched case expressions

select 
    case 
        when p.nickName = 'NA' then '<no nick name>' 
        else p.nickName end
from Person p

Note that when ... then ... could show up more than once.

For HQL's CASE expression involving one single when ... then ...  usage, it can be represented in MQL using $cond; otherwise $switch might be more appropriate without $cond cascading.