2 
var query = from c in db.Customers select c;
query = query.Where(c => c.Orders.Where(o => o.OrderItems.Where(oi => oi.SellerID == sellerID).Count() > 0).Count() > 0);

生成 SQL：

SELECT [t0].[CustomerID], [t0].[FirstName], [t0].[LastName], [t0].[Email], [t0].[Company], [t0].[BillingAddress1], [t0].[BillingAddress2], [t0].[ShippingAddress1], [t0].[ShippingAddress2], [t0].[Phone], [t0].[Mobile], [t0].[Fax], [t0].[CreateDT], [t0].[UpdateDT], [t0].[DefaultLanguage], [t0].[DefaultPaymentInfo], [t0].[Active], [t0].[Title], [t0].[LoginID], [t0].[ReferringLink], [t0].[CustomerIP], [t0].[CustomerCountryByIP], [t0].[VatID]
FROM [dbo].[Customers] AS [t0]
WHERE ((
    SELECT COUNT(*)
    FROM [dbo].[Orders] AS [t1]
    WHERE (((
        SELECT COUNT(*)
        FROM [dbo].[OrderItems] AS [t2]
        WHERE ([t2].[SellerID] = 2428) AND ([t2].[OrderID] = [t1].[OrderID])
        )) > 0) AND ([t1].[CustomerID] = [t0].[CustomerID])
    )) > 0

或者

var query = from c in db.Customers select c;
query = query.Where(c => c.Orders.Any(o => o.OrderItems.Any(oi => oi.SellerID == sellerID)));

生成 SQL：

SELECT [t0].[CustomerID], [t0].[FirstName], [t0].[LastName], [t0].[Email], [t0].[Company], [t0].[BillingAddress1], [t0].[BillingAddress2], [t0].[ShippingAddress1], [t0].[ShippingAddress2], [t0].[Phone], [t0].[Mobile], [t0].[Fax], [t0].[CreateDT], [t0].[UpdateDT], [t0].[DefaultLanguage], [t0].[DefaultPaymentInfo], [t0].[Active], [t0].[Title], [t0].[LoginID], [t0].[ReferringLink], [t0].[CustomerIP], [t0].[CustomerCountryByIP], [t0].[VatID]
FROM [dbo].[Customers] AS [t0]
WHERE EXISTS(
    SELECT NULL AS [EMPTY]
    FROM [dbo].[Orders] AS [t1]
    WHERE (EXISTS(
        SELECT NULL AS [EMPTY]
        FROM [dbo].[OrderItems] AS [t2]
        WHERE ([t2].[SellerID] = 2428) AND ([t2].[OrderID] = [t1].[OrderID])
        )) AND ([t1].[CustomerID] = [t0].[CustomerID])
    )

实际的执行计划看起来是一样的，但不管怎样，用什么更好呢？谢谢。

4

1 回答 1

1

有了EXISTS它，数据库立即清楚 1 条匹配记录就足够了。因此，一旦它找到任何东西，它就可以返回。有一个计数......可能不会使用优化。但是既然你说执行计划是一样的，我猜MS SQL明白它是如何优化查询的。

就我个人而言，我会选择存在，因为它准确地描述了你需要的东西。具有非零计数不太清楚恕我直言。

于 2010-08-31T10:20:30.197 回答