Cannot create indexed view

CREATE TABLE orders
(
   order_no INT N开发者_高级运维OT NULL PRIMARY KEY,
   prod_id INT NOT NULL,
   quantity INT
);

CREATE VIEW product_stats WITH SCHEMABINDING 
AS
SELECT a.prod_id, a.product_name, 
   (SELECT COUNT(*) FROM dbo.orders WHERE prod_id = a.prod_id) AS total FROM dbo.products a;

CREATE UNIQUE CLUSTERED INDEX [IDX_Order_Details_X] 
       ON product_stats (prod_id, total)

It complains: Column 'total' in view 'product_stats' cannot be used in an index or statistics or as a partition key because it does user or system data access.

DB is ms sql.

An indexed view cannot contain COUNT(*) or a subquery. See the "View Restrictions" section of this article.