Linq to SQL 语法查询....子查询 & in操作 & join

子查询

描述：查询订单数超过5的顾客信息

查询句法：

var 子查询 = from c in ctx.Customers

                   where

                       (from o in ctx.Orders group o by o.CustomerID into o where o.Count() > 5 select o.Key).Contains(c.CustomerID)

                   select c;

in 操作

描述：查询指定城市中的客户

查询句法：

        var in操作 = from c in ctx.Customers

                    where new string[] { "Brandenburg", "Cowes", "Stavern" }.Contains(c.City)

                    select c;

Join

描述：内连接，没有分类的产品查询不到

查询句法：

var innerjoin = from p in ctx.Products

                        join c in ctx.Categories

                        on p.CategoryID equals c.CategoryID

                        select p.ProductName;

描述：外连接，没有分类的产品也能查询到

查询句法：

var leftjoin = from p in ctx.Products

                       join c in ctx.Categories

                       on p.CategoryID equals c.CategoryID

                       into pro

                       from x in pro.DefaultIfEmpty()

                       select p.ProductName;