Into Clause VS Let Clause

1.Into Clause

    用来将select, join,或者group的结果存储到一个临时变量中。

    目的：在之后的查询中需要使用此结果。例如：

var developersGroupedByLanguage =
    from d in developers
    group d by d.Language into developersGrouped
    select new {
       Language = developersGrouped.Key,
       DevelopersCount = developersGrouped.Count()
    };
foreach (var group in developersGroupedByLanguage) {
     Console.WriteLine ("Language {0} contains {1} developers",
     group.Language, group.DevelopersCount);
}

2.Let Clause

    let子句将一个子表达式的结果存储到一个变量中。

    与SQL语句相比较，let子句产生的变量类似于计算列。即该列是通过对表中若干列进行某种计算获得的。例如对OrderDetail中的UnitPrice * Count得到某商品的总价。

var categoriesByProductsNumberQuery =
    from c in categories
    join p in products on c.IdCategory equals p.IdCategory
       into productsByCategory
    let ProductsCount = productsByCategory.Count()
    orderby ProductsCount
    select new { c.IdCategory, ProductsCount};
    foreach (var item in categoriesByProductsNumberQuery) {
	Console.WriteLine(item);
}