https://stackoverflow.com/questions/48723477/linq-one-to-many-relations-as-comma-separated-values

stack.48723477.cs

//Product ##
//ProductId
//ProductName
//IsDeleted(bit)
//
//ProductTag
//ProductTagId
//ProductId
//TagName
//IsDeleted(bit)
//
//ProductCategory
//ProductCategoryId
//ProductId
//CategoryName
//IsDeleted(bit)


void Main()
{
	var context = new Context();
	
	var query = from product in context.Products
				let tags = from tag in context.Tags
							where tag.IsDeleted
								&& tag.ProductId == product.ProductId
							select tag.TagName
				let categories = from category in context.Categories
							where category.IsDeleted
							 && category.ProductId == product.ProductId
							select category.CategoryName
				where !product.IsDeleted
				select new
				{
					Id = product.ProductId,
					Name = product.ProductName,
					Tags = string.Join(", ", tags)
					Categories = string.Join(" ", categories)
				};
	
}

class Context
{
	public IEnumerable<Product> Products { get; set; }
	public IEnumerable<ProductTag> Tags { get; set; }
	public IEnumerable<ProductCategory> Categories { get; set; }
}

class Product
{
	public int ProductId { get; set; }
	public string ProductName { get; set; }
	public bool IsDeleted { get; set; }
}

class ProductTag
{
	public int ProductId { get; set; }
	public int ProductTagId { get; set; }
	public string TagName { get; set; }
	public bool IsDeleted { get; set; }
}

class ProductCategory
{
	public int ProductCategoryId { get; set; }
	public int ProductId { get; set; }
	public string CategoryName { get; set; }
	public bool IsDeleted { get; set; }
}