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'.
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.
= 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
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
has a composite identifier consisting of
. 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
class, allowing us to express a join in the following way (where
AuditLog.item is a property mapped with
from AuditLog log, Payment payment
where log.item.class = 'Payment' and log.item.id = payment.id
payment.class would refer to the values of completely different database columns in the above query.