JPA Select Count Distinct [英] JPA Select Count Distinct

问题描述

我需要一个相对简单的查询，但JPA很难创建它。

I need a relatively simple query, but JPA makes it kind of hard to create it.

SQL变体如下所示：

The SQL variant would look like this:

SELECT COUNT(DISTINCT OrderID) AS DistinctOrders FROM Orders WHERE CustomerID=7;

我需要用一个变量设置 CustomerID 传递给我的方法。

I need to set the CustomerID with a variable that is passed to my method.

我找到了关于 CriteriaQuery的文档 distinct（）但我似乎无法将它们全部包装在一起。

I found documentation on CriteriaQuery distinct() but I can't seem to wrap it all together.

这是我到目前为止所做的：

This is what I have so far:

CriteriaBuilder cb = this.em.getCriteriaBuilder();
CriteriaQuery<Order> c = cb.createQuery( Order.class );
Root<Order> order = c.from( Order.class );
Path<String> customerID = order.get( "customerID" );
c.where( cb.equal( customerID, theId ) );

推荐答案

CriteriaBuilder cb = this.em.getCriteriaBuilder();
CriteriaQuery<Long> c = cb.createQuery(Long.class);//the query returns a long, and not Orders
Root<Order> order = c.from( Order.class );
//c.select(cb.countDistinct(order));//and this is the code you were looking for
c.select(cb.countDistinct(order.get("orderID")));//for counting distinct fields other than the primary key
Path<String> customerID = order.get( "customerID" );
c.where( cb.equal( customerID, theId ) );

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