Hibernate Criteria分组和聚合

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1.一个聚合错操作，例如cout(*)查询记录数

<span style="font-size:14px;">    Criteria crit = session.createCriteria(User.class);      crit.setProjection(Projections.rowCount());      int v = (Integer)crit.uniqueResult();  </span>

注意:setProjection（）可以多次使用，但是最后一次会覆盖前面的聚合操作
2.多个聚合操作

    ProjectionList proList = Projections.projectionList();      proList.add(Projections.rowCount());      proList.add(Projections.max("id"));      proList.add(Projections.sum("id"));      crit.setProjection(proList);      List results = crit.list();      Object[] arr = (Object[])results.get(0);      for(Object ob : arr){          System.out.println(ob.toString());        }

相当与执行:

select count(*),max(id),sum(id) from user

3.分组 group by

Criteria crit = session.createCriteria(User.class);  crit.setProjection(Projections.rowCount());  crit.setProjection(Projections.groupProperty("name"));  List results = crit.list();

相当于执行:

    select count(*) from user group by name

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