Question

给定一个列表IQueryables，如何在不在数据库中执行多个语句的情况下对每个列表进行求和？

 return queries
               .Sum(qy=> qy.Count());

上述工作正常，但会针对每个查询点击数据库。

Answer 1

您可以先使用Aggregate功能与Concat合并IQueryable，然后Count总计如下：

 return queries.Aggregate((x,y) => x.Concat(y)).Count()

Answer 2

从这个想法开始Sum(q1,q2) = q1.Concat(q2).Count()我测试了以下扩展程序：

public static class LinqExtensions
{
    public static IQueryable<object> ConcatAny<T,R>(this IQueryable<T> q1, IQueryable<R> q2)
    {
        return q1.Select(c=>(object)null).Concat(q2.Select(c=>(object)null));
    }

    public static IQueryable<object> ConcatAll(this IEnumerable<IQueryable<object>> queries)
    {
        var resultQuery = queries.First();
        foreach (var query in queries.Skip(1))
        {
            resultQuery = resultQuery.ConcatAny(query);
        }
        return resultQuery;
    }
}

我假设您有异构查询，例如IQueryable<T>，IQueryable<R>等等，您有兴趣计算所有行，无论其来源是什么。

所以你可以使用像

这样的扩展名

var q1 = Table1.AsQueryable();
var q2 = Table2.AsQueryable();
var q3 = Table3.AsQueryable();

var queries = new IQueryable<object>[] {q1,q2,q3}; // we use here the covariance feature

return queries.ConcatAll().Count();

生成的SQL可能如下所示

SELECT COUNT(*) AS [value]
FROM (
    SELECT NULL AS [EMPTY]
    FROM (
        SELECT NULL AS [EMPTY]
        FROM [Table1] AS [t0]
        UNION ALL
        SELECT NULL AS [EMPTY]
        FROM [Table2] AS [t1]
        ) AS [t2]
    UNION ALL
    SELECT NULL AS [EMPTY]
    FROM [Table3] AS [t3]
    ) AS [t4]

我认为虽然

非常有效

Answer 3

如果您使用的是Entity Framework，则可以使用名为EntityFramework.Extended的扩展程序。有一个名为Future Queries的内置扩展程序。这将允许您指定在下次进行数据库访问时应执行查询。

NuGet命令：

Install-Package EntityFramework.Extended

示例代码：

static void Main(string[] args)
{
    using (var context = new MyDbContext())
    {
        var modelSet1 = context.Models.Where(x => x.ModelId < 25).FutureCount();
        var modelSet2 = context.Models.Where(x => x.ModelId > 25 && x.ModelId < 32).FutureCount();
        var modelSet3 = context.Models.Where(x => x.ModelId > 32).FutureCount();

        var queries = new [] {modelSet1, modelSet2, modelSet3};

        var countQueries = queries.Sum(x => x.Value);
        Console.WriteLine(countQueries);
    }

    Console.ReadLine();
}

Answer 4

好的，迟到几分钟，但我明白了！这是代码：

    public static class LinqExtensions
    {
        public static int CountAll(this IEnumerable<IQueryable<object>> queries)
        {
            if (queries == null || !queries.Any())
            {
                throw new ArgumentException("Queries parameter cannot be null or empty");
            }
            Expression ex = Expression.Constant(0);
            foreach (var qy in queries)
            {
                // create count expression
                var expression = Expression.Call(
                  typeof(Queryable),
                  "Count",
                  new[] { qy.ElementType },
                  qy.Expression
                  );
                ex = Expression.Add(ex, expression);
            }
            return queries.First().Provider.Execute<int>(ex);
        }
    }

您将其用作queries.CountAll()，其中查询为IEnumerable<IQueryable<object>>，与Adrian的答案一样，甚至是简单的IEnumerable<IQueryable>。

以下是分析器的SQL结果示例：

exec sp_executesql N'SELECT @p0 + ((
    SELECT COUNT(*)
    FROM [A] AS [t0]
    WHERE [t0].[i1] >= @p1
    )) + ((
    SELECT COUNT(*)
    FROM [B] AS [t1]
    WHERE [t1].[i2] >= @p2
    )) + ((
    SELECT COUNT(*)
    FROM [C] AS [t2]
    WHERE [t2].[i3] >= @p3
    )) AS [value]',N'@p0 int,@p1 int,@p2 int,@p3 int',@p0=0,@p1=2,@p2=2,@p3=2

的代表

    var a = db.GetTable<A>();
    var b = db.GetTable<B>();
    var c = db.GetTable<C>();

    var q1 = a.Where(v => v.i1 >= 2);
    var q2 = b.Where(v => v.i2 >= 2);
    var q3 = c.Where(v => v.i3 >= 2);

    var queries = new IQueryable<object>[] {
        q1,q2,q3
    };

请注意，A，B和C是不同的对象/表，具有不同数量的属性/列，并且表达式是随机的过滤器。

IQueryable

4 个答案: