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from Event e, Attendee a where
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How do you know what object type the result list will contain The result list will contain a Cartesian product of the queried objects, which probably isn t what you want. To query on associated objects, you ll need to join them in the FROM clause.
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6.2.2 Joins
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You re probably familiar with SQL joins, which return data from multiple tables with a single query. You can think of HQL joins in a similar way, only you re joining object properties and associations instead of tables. If we want to return all Events that a specific Attendee is going to be attending, join the attendee property to the Event in the query:
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from Event e join e.attendees a where a.id=314
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You can join all associations (many-to-one and one-to-one), as well as collections, to the query s base object. (We refer to the base object in a query as the object listed in the FROM clause. In this case, the base object is the Event.) As the previous query shows, you can also assign an alias to joined associations and query on properties in the joined object. The naming convention for HQL aliases is to use a lowercase word, similar to the Java variable naming convention.
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Types of joins
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HQL has different types of joins, all but one of them taken from SQL. We summarize the join types in table 6.1.
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Querying objects with HQL
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Table 6.1 Join types Join Type inner join left [outer] join Rule Unmatched objects on either side of the join are discarded. All objects from the left side of the join are returned. If the object on the left side of the join has no matching object on the right side of the join, it is still returned. All objects from the right side of the join are returned. If the object on the right side of the join has no matching object on the left side of the join, it is still returned. All objects from either side of the join are returned, regardless of matching objects on the opposite side of the join. Used to retrieve an associated object or a collection of objects regardless of the outer-join or lazy property on the association. This join does not have a SQL counterpart.
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right [outer] join
full join
inner join fetch
Unless you specify left, right, or full as the prefix to the join statement, the default is to use inner joins. All of the joins in table 6.1 behave like their SQL counterparts, except for inner join fetch. Joining a lazy collection with inner join fetch will cause the collection to be returned as populated. For example:
from Event e inner join fetch e.speakers
returns all of the Event instances with populated collections of Speakers. Let s look at joining an object associated to the base object as a many-to-one:
from Event e join e.location l where l.name = :name
Joining the Location instance to the Event allows querying on the Location properties, and results in a more efficient SQL query. Let s say you had the following query:
Querying persistent objects
from Event e where e.location.name = :name and e.location.address.state = :state
Since you re walking the object graph twice, once for the Location name and again for the Location state, the query compiler will join the Location instance to the Event twice. Joining the Location to the Event in the FROM clause results in only one join and a more efficient query. Joined objects can also be returned in the SELECT clause of the HQL statement. The HQL SELECT clause is discussed next.
6.2.3 Selects
The SELECT clause allows you to specify a list of return values to be returned from a query. If you recall from chapter 1, selecting specific columns of data returned from a query is called projection. Possible return values include entire objects, specific object properties, and derived values from a query. Derived values include the results from various functions, such as min( ), max( ), and count( ). The SELECT clause does not force you to return entire objects. It s possible to return specific fields of objects, just as in SQL. Another interesting feature of HQL is the ability to return new objects from the selected values. We ll examine both features next.
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