如何在不加载内容的情况下在EntityFramework中计算行数？

我正在尝试确定如何使用EntityFramework 计算表上的匹配行。

问题在于每一行可能有许多兆字节的数据（在二进制字段中）。当然，SQL应该是这样的：

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

我可以加载所有行，然后找到Count：

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

但这是完全无效的。有没有更简单的方法？

编辑：谢谢，所有。我已将数据库从专用附件中移出，以便可以进行性能分析；这会有所帮助，但会引起我意想不到的混乱。

而我的真实数据是深一点，我会用卡车运送托盘的案件的资料 -我不想让卡车离开除非有至少一个项目在里面。

我的尝试如下所示。我不了解的部分是CASE_2从不访问DB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

CASE_1产生的SQL通过sp_executesql传递，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；从SQL中可以看到，Truck-Pallet和Pallet-Case关系是多对多的-尽管我认为这并不重要。我的真实物品是无形物品，很难描述，因此我更改了名称。]

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()

两者都生成相同的SQL查询。

我想你想要类似的东西

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

（编辑以反映评论）

据我了解，所选答案仍会加载所有相关测试。根据这个msdn博客，有更好的方法。

http://blogs.msdn.com/b/adonet/archive/2011/01/31/using-dbcontext-in-ef-feature-ctp5-part-6-loading-related-entities.aspx

特别

using (var context = new UnicornsContext())

    var princess = context.Princesses.Find(1);

    // Count how many unicorns the princess owns 
    var unicornHaul = context.Entry(princess)
                      .Collection(p => p.Unicorns)
                      .Query()
                      .Count();
}

这是我的代码：

IQueryable<AuctionRecord> records = db.AuctionRecord;
var count = records.Count();

确保将变量定义为IQueryable，然后在使用Count（）方法时，EF将执行类似

select count(*) from ...

否则，如果记录定义为IEnumerable，则生成的sql将查询整个表并计算返回的行。

好吧，即使SELECT COUNT(*) FROM TableSQL Server效率也相当低，尤其是在大型表上，因为SQL Server除了进行全表扫描（集群索引扫描）外，实际上什么也做不了。

有时，从数据库中知道大约多少行就足够了，在这种情况下，这样的语句就足够了：

SELECT 
    SUM(used_page_count) * 8 AS SizeKB,
    SUM(row_count) AS [RowCount], 
    OBJECT_NAME(OBJECT_ID) AS TableName
FROM 
    sys.dm_db_partition_stats
WHERE 
    OBJECT_ID = OBJECT_ID('YourTableNameHere')
    AND (index_id = 0 OR index_id = 1)
GROUP BY 
    OBJECT_ID

给定特定的表，这将检查动态管理视图并从中提取行数和表大小。通过汇总堆（index_id = 0）或聚集索引（index_id = 1）的条目来实现。

它快速，易于使用，但不能保证它是100％准确或最新的。但是在许多情况下，这已经足够了（并且减轻了服务器负担）。

也许对您也有用吗？当然，要在EF中使用它，您必须将其包装在存储的proc中，或使用直接的“执行SQL查询”调用。

马克

使用实体上下文的ExecuteStoreQuery方法。这样可以避免下载整个结果集并反序列化为对象以进行简单的行计数。

   int count;

    using (var db = new MyDatabase()){
      string sql = "SELECT COUNT(*) FROM MyTable where FkId = {0}";

      object[] myParams = {1};
      var cntQuery = db.ExecuteStoreQuery<int>(sql, myParams);

      count = cntQuery.First<int>();
    }

我认为这应该有效...

var query = from m in context.MyTable
            where m.MyContainerId == '1' // or what ever the foreign key name is...
            select m;

var count = query.Count();