The where clause allows you to narrow the list of instances returned. If no alias exists, you may refer to properties by name:

from Cat where name='Fritz'

If there is an alias, use a qualified property name:

from Cat as cat where cat.name='Fritz'

returns instances of Cat named 'Fritz'.

select foo
from Foo foo, Bar bar
where foo.startDate = bar.date

will return all instances of Foo for which there exists an instance of bar with a date property equal to the startDate property of the Foo. Compound path expressions make the where clause extremely powerful. Consider:

from Cat cat where cat.mate.name is not null

This query translates to an SQL query with a table (inner) join. If you were to write something like

from Foo foo
where foo.bar.baz.customer.address.city is not null

you would end up with a query that would require four table joins in SQL.

The = operator may be used to compare not only properties, but also instances:

from Cat cat, Cat rival where cat.mate = rival.mate

select cat, mate
from Cat cat, Cat mate
where cat.mate = mate

The special property (lowercase) id may be used to reference the unique identifier of an object. See Section 14.5, “Refering to identifier property” for more information.

from Cat as cat where cat.id = 123

from Cat as cat where cat.mate.id = 69

The second query is efficient. No table join is required!

Properties of composite identifiers may also be used. Suppose Person has a composite identifier consisting of country and medicareNumber. Again, see Section 14.5, “Refering to identifier property” for more information regarding referencing identifier properties.

from bank.Person person
where person.id.country = 'AU'
    and person.id.medicareNumber = 123456

from bank.Account account
where account.owner.id.country = 'AU'
    and account.owner.id.medicareNumber = 123456

Once again, the second query requires no table join.

Likewise, the special property class accesses the discriminator value of an instance in the case of polymorphic persistence. A Java class name embedded in the where clause will be translated to its discriminator value.

from Cat cat where cat.class = DomesticCat

You may also use components or composite user types, or properties of said component types. See Section 14.17, “Components” for more details.

An "any" type has the special properties id and class, allowing us to express a join in the following way (where AuditLog.item is a property mapped with <any>).

from AuditLog log, Payment payment
where log.item.class = 'Payment' and log.item.id = payment.id

Notice that log.item.class and payment.class would refer to the values of completely different database columns in the above query.