[Solved]-how to convert sql union to linq-LINQ,C#

There are the 101 Linq Samples - with two union samples Union1 and Union2

This Linq statement should get you the same results as your SQL: (it has for me on a test record-set)

var results = (from a in (from d in DiscountPromotions
            group d by d.BarCode into g
            select new { 
                BarCode = g.Key,
                AmountTaken = g.Sum(p => p.AmountTaken)
                }).Union(from i in ItemSaleTransactions
            group i by i.BarCode into o
            select new { 
                BarCode = o.Key,
                AmountTaken = o.Sum(i => i.AmountTaken)
                }) group a by a.BarCode into b
                select new {
                    BarCode = b.Key,
                    AmountTaken = b.Sum(c => c.AmountTaken)
                });

Here's an example of a generic union, without regard to the scenario you posted:

var something =
                (from e in _repository
                 select new { e.Property1, e.Property2 }).Union(
                (from e in _repository
                 select new { e.Property1, e.Property2 }));

Three useful Linq concepts operating on sets. Given set c and set e:

Concat gives you everything in c or e:

(From c In db.Customers Select c.Phone).Concat( _
             From c In db.Customers Select c.Fax).Concat( _
             From e In db.Employees Select e.HomePhone)

(From c In db.Customers _
            Select Name = c.CompanyName, Phone = c.Phone).Concat(From e In db.Employees _
            Select Name = e.FirstName & " " & e.LastName, Phone = e.HomePhone)

Union also gives you everything in c and e, but removes any duplicates:

(From c In db.Customers _
        Select c.Country).Union(From e In db.Employees _
        Select e.Country)

Except gives you everything in c that is not in e:

(From c In db.Customers _
             Select c.Country).Except(From e In db.Employees Select e.Country)