本文介绍了Hibernate中如何通过子查询表达式映射视图，即使在无法创建数据库视图的情况下也能实现。通过示例展示了如何定义只读和不可变的实体以同步数据，并确保自动刷新及防止查询返回陈旧数据。
摘自[url]http://www.hibernate.org/hib_docs/reference/en/html/mapping.html[/url]
There is no difference between a view and a base table for a Hibernate mapping, as expected this is transparent at the database level (note that some DBMS don't support views properly, especially with updates). Sometimes you want to use a view, but can't create one in the database (ie. with a legacy schema). In this case, you can map an immutable and read-only entity to a given SQL subselect expression:
<class name="Summary">
<subselect>
select item.name, max(bid.amount), count(*)
from item
join bid on bid.item_id = item.id
group by item.name
</subselect>
<synchronize table="item"/>
<synchronize table="bid"/>
<id name="name"/>
...
</class>
Declare the tables to synchronize this entity with, ensuring that auto-flush happens correctly, and that queries against the derived entity do not return stale data. The <subselect> is available as both as an attribute and a nested mapping element.
There is no difference between a view and a base table for a Hibernate mapping, as expected this is transparent at the database level (note that some DBMS don't support views properly, especially with updates). Sometimes you want to use a view, but can't create one in the database (ie. with a legacy schema). In this case, you can map an immutable and read-only entity to a given SQL subselect expression:
<class name="Summary">
<subselect>
select item.name, max(bid.amount), count(*)
from item
join bid on bid.item_id = item.id
group by item.name
</subselect>
<synchronize table="item"/>
<synchronize table="bid"/>
<id name="name"/>
...
</class>
Declare the tables to synchronize this entity with, ensuring that auto-flush happens correctly, and that queries against the derived entity do not return stale data. The <subselect> is available as both as an attribute and a nested mapping element.
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