单个LINQ查询中的多个总和会触发多个SQL查询

Question

As a simple example let's assume we have an IQueryable named "myQueryable" with 3 columns Id, Value1 and Value2 and we write the following LINQ expression:

var result = myQueryable.Select(g => new 
{ 
    Value1Sum = g.Sum(b => b.Value1),
    Value2Sum = g.Sum(b => b.Value2)                                  
});

This will fire 2 different queries, one for each sum and then return both of them as an object like

{ Value1Sum = x , Value2Sum = y }

Is it possible to build the LINQ Expression in such a way that only one query is fired for both sums? After all they are both calculated on the basis of the same dataset..

To make it clearer, I'd expect a query like this:

SELECT SUM(Value1), SUM(Value2) FROM MyTable

but LINQ generates two queries like this(can't post the actual queries fired for security reasons):

exec sp_executesql N'SELECT 
[GroupBy1].[A1] AS [C1]
FROM ( SELECT 
    SUM([Extent1].[Value1]) AS [A1]
    FROM [mo].[MyTable] AS [Extent1]) AS [GroupBy1]

..same for Value2. Please note that even if it calls the inner select "GroupBy1" there is no grouping anywhere (also in the actual queries)

Answer 1

Yes, you can do that in a single query to your database.

First, get all the values you need.

var values = MyQueryable.Select(item => new { Value1 = item.Value1 , Value2 = item.Value2 })
.ToList();

You use ToList() to force the query to execute. The Select() is only to minimize the data you query. You might as well use

var values = MyQueryable.ToList();

This is your only trip to the database.

Now that you have all data locally, you can aggregate these values.

Tuple<int, int> sums = values.Aggregate(
new Tuple<int, int>(0,0),
(tuple, item) => new Tuple<int, int>(tuple.Item1 + item.Value1, tuple.Item2 + item.Value2));

Afterwards you have a tuple sums where sums.Item1 represents the sum of all Value1 and sums.Item2 represents the sum of all Value2 respectively.

Can I use Aggregate directly on my IQueryable<T> ?

Unfortunately, no. Aggregate() is not supported in LINQ to Entities , so you cannot use it on IQueryable<T> directly. This is why you first need to get the data locally so you have an IEnumerable<T> . Then you can aggregate the results using LINQ to objects . See this StackOverflow answer on the distinction between the two.