LINQ: combining join and group by

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I have a query that combines a join and a group, but I have a problem. The query is like:

 var result = from p in Products                         
 join bp in BaseProducts on p.BaseProductId equals bp.Id                    
 group p by p.SomeId into pg                         
 select new ProductPriceMinMax { 
       SomeId = pg.FirstOrDefault().SomeId, 
       CountryCode = pg.FirstOrDefault().CountryCode, 
       MinPrice = pg.Min(m => m.Price), 
       MaxPrice = pg.Max(m => m.Price),
       BaseProductName = bp.Name  <------ can't use bp. 

As you see, it joins the Products table with the BaseProducts table, and groups on an id of the Product table. But in the resulting ProductPriceMinMax, I also need a property of the BaseProducts table: bp.Name, but it doesn't know bp.

Any idea what I'm doing wrong?


Once you've done this

group p by p.SomeId into pg  

you no longer have access to the range variables used in the initial from. That is, you can no longer talk about p or bp, you can only talk about pg.

Now, pg is a group and so contains more than one product. All the products in a given pg group have the same SomeId (since that's what you grouped by), but I don't know if that means they all have the same BaseProductId.

To get a base product name, you have to pick a particular product in the pg group (As you are doing with SomeId and CountryCode), and then join to BaseProducts.

var result = from p in Products                         
 group p by p.SomeId into pg                         
 // join *after* group
 join bp in BaseProducts on pg.FirstOrDefault().BaseProductId equals bp.Id         
 select new ProductPriceMinMax { 
       SomeId = pg.FirstOrDefault().SomeId, 
       CountryCode = pg.FirstOrDefault().CountryCode, 
       MinPrice = pg.Min(m => m.Price), 
       MaxPrice = pg.Max(m => m.Price),
       BaseProductName = bp.Name  // now there is a 'bp' in scope

That said, this looks pretty unusual and I think you should step back and consider what you are actually trying to retrieve.

Perform grouped joins (LINQ in C#), The group join is useful for producing hierarchical data structures. It pairs each element from the first collection with a set of correlated elements  Example - Group join to create XML. Group joins are ideal for creating XML by using LINQ to XML. The following example is similar to the previous example except that instead of creating anonymous types, the result selector function creates XML elements that represent the joined objects. C#.

We did it like this:

from p in Products                         
join bp in BaseProducts on p.BaseProductId equals bp.Id                    
where !string.IsNullOrEmpty(p.SomeId) && p.LastPublished >= lastDate                         
group new { p, bp } by new { p.SomeId } into pg    
let firstproductgroup = pg.FirstOrDefault()
let product = firstproductgroup.p
let baseproduct = firstproductgroup.bp
let minprice = pg.Min(m => m.p.Price)
let maxprice = pg.Max(m => m.p.Price)
select new ProductPriceMinMax
SomeId = product.SomeId,
BaseProductName = baseproduct.Name,
CountryCode = product.CountryCode,
MinPrice = minprice, 
MaxPrice = maxprice

EDIT: we used the version of AakashM, because it has better performance

Grouped Joins in LINQ, In this article I will try to explain grouped joins in LINQ. We have 4 types of joins for Language Integrated Query (LINQ), these Next Recommended Article Joining Multiple Data-Sources Using "Concat" Key in LINQ: Part 7. In Visual Basic, LINQ provides two options for performing an INNER JOIN: an implicit join and an explicit join. An implicit join specifies the collections to be joined in a From clause and identifies the matching key fields in a Where clause. Visual Basic implicitly joins the two collections based on the specified key fields.

I met the same problem as you.

I push two tables result into t1 object and group t1.

 from p in Products                         
  join bp in BaseProducts on p.BaseProductId equals bp.Id
  select new {
  } into t1
 group t1 by t1.p.SomeId into g
 select new ProductPriceMinMax { 
  SomeId = g.FirstOrDefault().p.SomeId, 
  CountryCode = g.FirstOrDefault().p.CountryCode, 
  MinPrice = g.Min(m => m.bp.Price), 
  MaxPrice = g.Max(m => m.bp.Price),
  BaseProductName = g.FirstOrDefault().bp.Name

GroupJoin - Joining Operator, This tutorial explains how to use GroupJoin in linq. The GroupJoin operator joins two sequences based on key and groups the result by matching key and then  After grouping by your Key you should use SelectMany to merge all the collection you got from the different records. After doing so you can call Distinct. var result = pageData.GroupBy(item => item.Key) .Select(group => new { Key = group.Key, Values = group.SelectMany(item => item.Value).Distinct().ToList() }).ToList();

Joining, Grouping and GroupJoining in LINQ – Hryniewski.NET, Arranging collections by some sort of key or other common value with LINQ is easy task. Just think about basic SQL joins with foreign keys, ids  While working with Entity Framework LINQ people ususally get struck on how to use joins, group by cluase, count etc.. Joins take two source sequences as input. The join clause compares the specified keys for equality by using equals keyword. Their are different types of joins; depending upon our senario, we can use any one. Example

[SOLVED], From data you provided, I think query should look like from z in db.Zeiterfassung join f in db.Firma on z.FirmenID equals f.ID join t in db. Avoiding the lambda expressions (almost) and using a "purely" LINQ way: var sums = from foo in list group foo by foo.GroupId into groupings orderby groupings.Key ascending select new { GroupId = groupings.Key, ValueA = groupings.Sum(g => g.ValueA), ValueB = groupings.Sum(g => g.ValueB) };

[Solved] Linq with multiple table joins, ordering and grouping , see these also 1) how-to-join-two-tables-using-linq-to-sql/[^] 2) Linq-Group By[^] 3) join-and-group-by-linq-to-entities/[^] Joining tables with group by and order by; Join two tables related by a single column primary key or foriegn key pair; Join two tables related by a composite primary key or foriegn key pair; Join three or more tables based on a parent-child relationship; Using a where clause to join tables based on nonkey columns

  • This is better than our solution :) Thanks.
  • @AakashM Thanks. But I am fetching a problem with groupVariable.FirstOrDefault().column_name. When I am going to access the IQueryable variable Result then the error occurred which heading is "entitycommandexecutionexception was unhandled by user code" with body "An error occurred while executing the command definition. See the inner exception for details." I can't understand what is happening.
  • @MuhammadAshikuzzaman sounds to me like you need to Ask a new question. Be sure to include all the details!