计算HQL查询中的组数? [英] Count the number of groups in HQL Query?

问题描述

我有以下HQL查询:

select c.device from Choice c 
group by c.device

现在我要计算结果中的组数，而不是每个组中的设备数.

Now I want to count the number of groups in the result not the number of devices per group.

我尝试过:

select count(distinct c.device) from Choice c 
    group by c.device

但是，这给出了每个组中不同设备的数量.这类似于 [2,3,4] .但是我需要 2 + 3 + 4 .

but this give the number of distinct devices in each group. This is something like [2,3,4]. But I need 2+3+4.

如何获得HQL的组数?

推荐答案

您必须对HQL不支持的计数进行计数.

You would have to do a count of a count, which is not supported in HQL.

在SQL中，它看起来像这样:

In SQL it would look something like this:

select count(innerQuery.counted)
from (select count(d.id) as counted
      from Choice c inner join Device d
      on c.device_id = d.id
      group by d.id) as innerQuery

在上面的示例中，外部查询从返回组的子查询中选择.然后，外部查询将对生成的 counted 列进行计数.

In the example above the outer query selects from a sub-query which returns the groups. The outer query then does a count on the generated counted column.

一种替代方法是进行计数，然后获取列表的大小.

An alternative is to do a count and then get the size of the list.

Choice.executeQuery('select count(c.device) from Choice c group by c.device').size()

由于计数是在客户端进行的，因此会降低性能.

There's a performance penalty because the counting is done on the client-side.

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