Linq Group by Multiple Fields -Syntax Help

LINQ grouping multiple fields only if one of the fields is a specific value

The GroupBy returns an anonymous type with 2 fields. The values inside the returned object can be anything. In this case they can be altered if the category is "01": myItemList.GroupBy(a=> new {a.Category, Vendor= a.Category == "01" ? null : a.Vendor}) To keep in line with your current code, the property is named Vendor, but can be any name.

In your current code:

List<MyItem> groupedItems = myItemList
    .GroupBy(a => new {a.Category, Vendor = a.Category == "01" ? null : a.Vendor})
    .Select(b => new MyItem
    {
        Vendor = b.First().Vendor,
        Cost = b.Sum(c => c.Cost),
        Category = b.First().Category
    })
    .ToList();

Or with a slight alteration, instead of the First() value of a group, you can reuse the key (and implement the N\A in one go)

List<MyItem> groupedItems = myItemList
      .GroupBy(a=> new {a.Category, Vendor= a.Category == "01" ? "N/A" : a.Vendor})
      .Select(b=> new MyItem{
            Vendor = b.Key.Vendor,
            Cost = b.Sum(c => c.Cost),
            Category = b.Key.Category
       }).ToList();

LINQ GroupBy for multiple Column

You do not need to group by multiple columns. Based on sample data you are only grouping by one field, CustomerID.

var objCustomerBuildingMappingResult = objTicketsForTheDayInfo.TicketsForTheDay
.GroupBy(l => l.CustomerID)
.Select(grp => new CustomerBuildingMapping
        {
            CustomerID = grp.Key,
            LeadId = long.Parse(grp.Key) + 1000,
            BuildingInfo = grp.Select(l => new BuildingInfo {
                BuildingID = l.BuildingID,
                TicketNumber = l.Ticket,
                Amount = l.Amount,
                BuildingName = l.BuildingName
            }).ToList(),
        }).ToList();

Linq Group By multiple fields and Flatten the list

It seems to me that the only problem is that you're grouping twice. I'd expect this to work:

var query = from rating in ratings
            group rating by rating.ArticleTitle into g
            select new
            {
                Title = g.Key,
                Yes = g.Count(r => r.Useful),
                No = g.Count(r => !r.Useful)
            };

Or not in query expression form:

var query = ratings.GroupBy(r => r.ArticleTitle,
                            (key, rs) => new
                            {
                                Title = key,
                                Yes = rs.Count(r => r.Useful),
                                No = rs.Count(r => !r.Useful)
                            });

How to GroupBy and Order by multiple fields with LINQ

If you need to filter out duplicates, try the following query:

var dtUniqueResults = dtResult.AsEnumerable()
    .GroupBy(n => new 
        { 
            Id = n.Field<string>("id"),
            Store = n.Field<string>("store"),
            Sku = n.Field<string>("sku")
        }
    )
    .SelectMany(g => g.Take(1)) // get from group only one record
    .CopyToDataTable();

Linq Query to Group By Multiple Columns But Get Additional Properties

When you use GroupBy in LINQ, you get an enumerable of a class which implements interface IGrouping<TKey,TElement> - the TElement representing the individual, and original, elements that have been grouped.

Therefore you would group like so

 var grouped = myList.GroupBy(x => new {x.StoreName,x.Address1});

Which would give you an enumerable list of IGrouping<anonymous,Store> (Assuming the original object was called Store). The anonymous is an anonymous object with 2 properties StoreName and Address1.

Now, each item in the enumerable is itself enumerable of Store, and you can treat that exactly as you would any other enumerable - you can take the First().ID if you wish, and you could project the whole lot back out to an enumerable if thats what you want

var result = myList.GroupBy(x => new {x.StoreName,x.Address1});
                   .Select(g => new {
                                  g.Key.StoreName, 
                                  g.Key.Address1, 
                                  FirstID = g.First().ID
                                });

LINQ group by with multiple fields and aggregate function

In the last select statement the only parameter from the query you are accessible to is grp; where each group is IGrouping; each group is represented by a key and an IEnumerable which is in your case : IEnumerable<a>. Having said that; you can replace the line total = a.kkkk.Count() with total = grp.SelectMany(v=> v.kkkk).Count() OR total = grp.Sum(a => a.kkkk.Count()) to resolve the issue.
This way total represents the summation of kkkk counts in each a in the group.

Linq to Entities grouping on multiple fields with an Or operator

Try this:-

 var query = from ord in orders
                        group ord by new { ord.Country, ord.PostCode } into g
                        select new
                            {
                                Country = g.Key.Country,
                                PostCode = g.Key.PostCode,
                                Orders = g.GroupBy(x => x.OrderId)
                                          .GroupBy(i => i.Count() > 1 ?
                                              new { OrderID = i.Key, Email = default(string) }
                                              : new { OrderID = default(int), i.First().Email })
                                          .Select(o => o.Count() == 1 ?
                                               new { o.Key, Orders = o.First().ToList() }
                                               : new { o.Key, Orders = o.Select(z => z.First()).ToList() })
                            };

Here is the complete Working Fiddle.

Linq group by with multiple columns

It seems like you need grouping on Country name as it is repeating in the data but with different id (which is strange) in that case you need to group and select the first one :

var result1 = myRepoOutput.GroupBy(x => x.Country)
                          .Select(b => b.First());

Returning multiple fields with LINQ GroupBy on a datatable

Create an anonymous type to group on:

var query = ds.Tables[0].AsEnumerable()
            .GroupBy(r => new
            {
                state = r.Field<string>("state"),
                name = r.Field<string>("name"),
            })
            .Select(grp => new
            {
                name = grp.Key.name,
                state = grp.Key.state,
                Count = grp.Count()
            })
            .OrderBy(o => o.state)
            .ToList();

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