Hibernate ProjectionList 用于从具有列值限制的表中获取计数

Question

class Books{
    private int bookId; //primary key
    private String bookName;
    Private String category;
}

考虑上面的模型类（书籍），它的类别变量为 A、B、C。

它存储在表 - 'books' 中，列名为 'book_id' 、'book_name' 、 'category'（类别值将是 A 或 B 或 C）。

使用Hibernate ProjectionList，我想拥有

1.count of book_id(from DB table) with category value as A 
2.count of book_id(from DB table) with category value as B 
3.count of book_id(from DB table) with category value as C

将其分配给 3 个单独的 int 值 countA,countB,countC

Answer 1

                Criteria cr = session.createCriteria(Book.class);
                ProjectionList projList = Projections.projectionList();        
                projList.add(Projections.groupProperty("category"));
                projList.add(Projections.count("category").as("count"));
                cr.setProjection(projList);
                List<Object[]> rows = cr.list();
                if(rows.size()>0){
                    for(Object[] itr : rows){
                        if("A".equalsIgnoreCase(((String) itr[0]).trim())){
                            int countA = (long)itr[1];
                        }
                        if("B".equalsIgnoreCase(((String) itr[0]).trim())){
                            int countB = (long)itr[1];
                        }
                        if("C".equalsIgnoreCase(((String) itr[0]).trim())){
                            int countC = (long)itr[1];
                        }
                    }
                }

Answer 2

只是为了展示其中一种方法，在How do we count rows using Hibernate? 中链接并与 Doing an "IN" query with Hibernate

Integer count = (Integer) session.CreateQuery("select count(*) from Books where category = ?1")
     .query.setParameter(1, "A")
     .uniqueResult();