java 休眠本机查询，计数
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hibernate native query, count
提问by Ilya
Possible Duplicate:
How do we count rows using Hibernate?
可能的重复：
我们如何使用 Hibernate 计算行数？
I want count number of records by criteria in database.
I try use next query
我想按数据库中的条件计算记录数。
我尝试使用下一个查询
String queryString = "SELECT Count(*) FROM my_table";
Query query = entityManager.createNativeQuery(queryString);
but there is no method to execute this Query queryand get result.
I know, that I can count records using
但是没有方法可以执行此操作Query query并获得结果。
我知道，我可以使用
String queryString = "SELECT * FROM my_table";
Query query = entityManager.createNativeQuery(queryString);
query.getResultList().size();
so question, is query with Count(*)more performance, and if yes, then how can I execute query with Count(*)?
所以问题是，查询是否具有Count(*)更高的性能，如果yes，那么我该如何执行查询Count(*)？
回答by Adam Dyga
You can execute your first query by calling Query.getSingleResult(), eg.
您可以通过调用来执行您的第一个查询Query.getSingleResult()，例如。
String queryString = "SELECT Count(*) FROM my_table";
Query query = entityManager.createNativeQuery(queryString);
System.out.println(query.getSingleResult());
If you want to assign the count to a variable you need to cast it to proper type (it can depend on DB but most likely it's Long). The second query is very inefficient because Hibernate needs to retrieve entire table from DB, analyze it and then count all rows.
如果要将计数分配给变量，则需要将其强制转换为正确的类型（它可能取决于 DB，但很可能是 Long）。第二个查询非常低效，因为 Hibernate 需要从数据库中检索整个表，对其进行分析，然后对所有行进行计数。
回答by RobertG
Have you tried calling query.getSingleResult()?
您是否尝试过调用 query.getSingleResult()？
