14.9. The where clause
The
where
clause allows you to refine the list of instances returned. If no alias exists, you can 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'
This returns instances of
Cat
named 'Fritz'.
The following query:
select foo from Foo foo, Bar bar where foo.startDate = bar.date
returns all instances of
Foo
with 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 the following:
from Cat cat where cat.mate.name is not null
This query translates to an SQL query with a table (inner) join. For example:
from Foo foo where foo.bar.baz.customer.address.city is not null
would result in a query that would require four table joins in SQL.
The
=
operator can 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
can be used to reference the unique identifier of an object. See Section 14.5, “Referring 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 and does not require a table join.
Properties of composite identifiers can also be used. Consider the following example where
Person
has composite identifiers consisting of country
and medicareNumber
:
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 does not require a table join.
See Section 14.5, “Referring to identifier property” for more information regarding referencing identifier properties)
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 can also use components or composite user types, or properties of said component types. See Section 14.17, “Components” for more information.
An "any" type has the special properties
id
and class
that allows you 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
The
log.item.class
and payment.class
would refer to the values of completely different database columns in the above query.