C# - Multiple LINQ query into One?

Question

I have used two LINQ queries to manipulate a datatable and return the data that I want. Effectively, the table has the following columns:

Group Animal Numbers

There can be multiple Animals within a single group, and multiple numbers assigned to a single animal. Initially I want to calculate the range of the numbers for each specific animal, and then calculate an average range for each group. With the output being the group number and then the average range.

I have the following two LINQ queries, that do work, however my question is, can this be merged into a single query, or , as they are distinct operations, should they be kept seperate for readability and testability?

 //Calculate range/delta for each number per animal var query = from data in Data.AsEnumerable() group data by new { animal = data.Field<string>("animal"), gp = data.Field<string>("group") } into g orderby g.Key.gp select new { animal = g.Key.animal, gp = g.Key.gp, delta = g.Max(c => Convert.ToDouble(c.Field<string>("numbers"))) - g.Min(c => Convert.ToDouble(c.Field<string>("numbers"))) }; //calculate average range/delta per group var temp = from q in query group q by q.gp into g select new { gp = g.Key, average = g.Average(c => c.delta) };

Thanks.

neontapir · Accepted Answer · 2011-03-22 15:03:26Z

5

Because you aren't forcing the first query to evaluate with a method like ToArray(), there's no advantage to combining these queries. I would leave them separate for readability.

answered Mar 22, 2011 at 15:03

neontapir

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3 Comments

Brian Cauthon Over a year ago

Actually he is forcing the query to be evaluated by calling AsEnumerable() in the first query.

neontapir Over a year ago

AsEnumerable doesn't force query evaluation. <msdn.microsoft.com/en-us/library/bb335435.aspx>

Brian Cauthon Over a year ago

The last sentence in the remarks section in the article you linked to is important. True, calling AsEnumerable() doesn't immeadiately enumerate the Data entities. It does however effect which implementation of group is used, Table<T> vs IEnumerable<T>. This can have a HUGE impact on the SQL generated when it finally is evaluated. For example, a where would not be evaluated on the server side. All entities would be retrieved and the where would be done using Linq-to-Objects.

dtb · Accepted Answer · 2011-03-22 15:12:59Z

You could first group the data by group and then, in a nested query, by animal to calculate the average of the deltas:

var result = from data in Data.AsEnumerable() group data by data.Field<string>("group") into g select new { gp = g.Key, average = ( from item in g group item by data.Field<string>("animal") into f select f.Max(c => double.Parse(c.Field<string>("numbers"))) - f.Min(c => double.Parse(c.Field<string>("numbers"))) ).Average() };

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