此内容没有您所选择的语言版本。

14.16. Tips & Tricks


You can count the number of query results without returning them:
( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue()
Copy to Clipboard Toggle word wrap
To order a result by the size of a collection, use the following query:
select usr.id, usr.name
from User as usr
    left join usr.messages as msg
group by usr.id, usr.name
order by count(msg)
Copy to Clipboard Toggle word wrap
If your database supports subselects, you can place a condition upon selection size in the where clause of your query:
from User usr where size(usr.messages) >= 1
Copy to Clipboard Toggle word wrap
If your database does not support subselects, use the following query:
select usr.id, usr.name
from User usr
    join usr.messages msg
group by usr.id, usr.name
having count(msg) >= 1
Copy to Clipboard Toggle word wrap
As this solution cannot return a User with zero messages because of the inner join, the following form is also useful:
select usr.id, usr.name
from User as usr
    left join usr.messages as msg
group by usr.id, usr.name
having count(msg) = 0
Copy to Clipboard Toggle word wrap
Properties of a JavaBean can be bound to named query parameters:
Query q = s.createQuery("from foo Foo as foo where foo.name=:name and foo.size=:size");
q.setProperties(fooBean); // fooBean has getName() and getSize()
List foos = q.list();
Copy to Clipboard Toggle word wrap
Collections are pageable by using the Query interface with a filter:
Query q = s.createFilter( collection, "" ); // the trivial filter
q.setMaxResults(PAGE_SIZE);
q.setFirstResult(PAGE_SIZE * pageNumber);
List page = q.list();
Copy to Clipboard Toggle word wrap
Collection elements can be ordered or grouped using a query filter:
Collection orderedCollection = s.createFilter( collection, "order by this.amount" ).list();
  Collection counts = s.createFilter( collection, "select this.type, count(this) group by this.type" ).list();
Copy to Clipboard Toggle word wrap
You can find the size of a collection without initializing it:
( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue();
Copy to Clipboard Toggle word wrap
返回顶部
Red Hat logoGithubredditYoutubeTwitter

学习

尝试、购买和销售

社区

关于红帽文档

通过我们的产品和服务,以及可以信赖的内容,帮助红帽用户创新并实现他们的目标。 了解我们当前的更新.

让开源更具包容性

红帽致力于替换我们的代码、文档和 Web 属性中存在问题的语言。欲了解更多详情,请参阅红帽博客.

關於紅帽

我们提供强化的解决方案,使企业能够更轻松地跨平台和环境(从核心数据中心到网络边缘)工作。

Theme

© 2025 Red Hat