c# – 如何在EntityFramework中的COUNT行,而不加载内容?

我试图确定如何计数表上使用EntityFramework匹配的行。

问题是每行可能有多个兆字节的数据(在二进制字段中)。当然SQL会是这样的:

SELECT COUNT(*) FROM [MyTable] WHERE [fkID] = '1';

我可以加载所有的行,然后找到计数与:

var owner = context.MyContainer.Where(t => t.ID == '1');
owner.MyTable.Load();
var count = owner.MyTable.Count();

但这是非常低效的。有更简单的方法吗?

编辑:谢谢,所有。我已经从一个私人的连接移动数据库,所以我可以运行性能分析;这有助于但导致我没有想到的混乱。

而我的真实数据有点深,我会用卡车搬运货物的箱子 – 我不想让卡车离开,除非有至少一个项目。

我的尝试如下所示。我没有得到的部分是CASE_2从来没有访问数据库服务器(MSSQL)。

var truck = context.Truck.FirstOrDefault(t => (t.ID == truckID));
if (truck == null)
    return "Invalid Truck ID: " + truckID;
var dlist = from t in ve.Truck
    where t.ID == truckID
    select t.Driver;
if (dlist.Count() == 0)
    return "No Driver for this Truck";

var plist = from t in ve.Truck where t.ID == truckID
    from r in t.Pallet select r;
if (plist.Count() == 0)
    return "No Pallets are in this Truck";
#if CASE_1
/// This works fine (using 'plist'):
var list1 = from r in plist
    from c in r.Case
    from i in c.Item
    select i;
if (list1.Count() == 0)
    return "No Items are in the Truck";
#endif

#if CASE_2
/// This never executes any SQL on the server.
var list2 = from r in truck.Pallet
        from c in r.Case
        from i in c.Item
        select i;
bool ok = (list.Count() > 0);
if (!ok)
    return "No Items are in the Truck";
#endif

#if CASE_3
/// Forced loading also works, as stated in the OP...
bool ok = false;
foreach (var pallet in truck.Pallet) {
    pallet.Case.Load();
    foreach (var kase in pallet.Case) {
        kase.Item.Load();
        var item = kase.Item.FirstOrDefault();
        if (item != null) {
            ok = true;
            break;
        }
    }
    if (ok) break;
}
if (!ok)
    return "No Items are in the Truck";
#endif

并且从CASE_1生成的SQL通过sp_executesql管道,但是:

SELECT [Project1].[C1] AS [C1]
FROM   ( SELECT cast(1 as bit) AS X ) AS [SingleRowTable1]
LEFT OUTER JOIN  (SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(cast(1 as bit)) AS [A1]
        FROM   [dbo].[PalletTruckMap] AS [Extent1]
        INNER JOIN [dbo].[PalletCaseMap] AS [Extent2] ON [Extent1].[PalletID] = [Extent2].[PalletID]
        INNER JOIN [dbo].[Item] AS [Extent3] ON [Extent2].[CaseID] = [Extent3].[CaseID]
        WHERE [Extent1].[TruckID] = '....'
    )  AS [GroupBy1] ) AS [Project1] ON 1 = 1

[我真的没有卡车,司机,托盘,箱子或物品;正如你可以从SQL看到的Truck-Pallet和Pallet-Case关系是多对多 – 虽然我不认为重要。我的现实对象是无形的,更难描述,所以我改变了名字。

查询语法:

var count = (from o in context.MyContainer
             where o.ID == '1'
             from t in o.MyTable
             select t).Count();

方法语法:

var count = context.MyContainer
            .Where(o => o.ID == '1')
            .SelectMany(o => o.MyTable)
            .Count()

两者生成相同的SQL查询。

http://stackoverflow.com/questions/890381/how-to-count-rows-within-entityframework-without-loading-contents

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