Sql LINQ聚合/和分组问题

我很难理解如何将传统的SQL聚合查询转换为LINQ查询。基本数据转储的工作原理如下：

    Dim result =
    (From i As Models.InvoiceDetail In Data.InvoiceDetails.GetAll
    Join ih As Models.InvoiceHeader In Data.InvoiceHeaders.GetAll On i.InvoiceHeaderID Equals ih.ID
    Join p As Models.Product In Data.Products.GetAll On i.ProductID Equals p.ID
    Join pg As Models.ProductGroup In Data.ProductGroups.GetAll On p.ProductGroupID Equals pg.ID
    Join gl As Models.GLAccount In Data.GLAccounts.GetAll On pg.GLAccountSellID Equals gl.ID
    Where (gl.ID = GLID)
    Select ih.Period,i.ExtendedValue)

---

我真正需要得到的是I.Period（从1到12的值）和I.ExtendedValue的相应聚合值。当我尝试对ih进行分组时，我会发现一些错误，比如我超出了范围/上下文，我不确定如何进行分组。

为了简单起见，请尝试将其有效地拆分为两个不同的查询。例如：

From result In
    (From i As Models.InvoiceDetail In Data.InvoiceDetails.GetAll
    Join ih As Models.InvoiceHeader In Data.InvoiceHeaders.GetAll On i.InvoiceHeaderID Equals ih.ID
    Join p As Models.Product In Data.Products.GetAll On i.ProductID Equals p.ID
    Join pg As Models.ProductGroup In Data.ProductGroups.GetAll On p.ProductGroupID Equals pg.ID
    Join gl As Models.GLAccount In Data.GLAccounts.GetAll On pg.GLAccountSellID Equals gl.ID
    Where (gl.ID = GLID) And (ih.FinancialYear = FinancialYear)
    Select ih.Period, i.ExtendedValue)
Group By result.Period Into Sum(result.ExtendedValue)