Linq实现t-Sql的各种连接

在ORM框架大行其道的今天，对于.net行业的人，想要学好EF，那Linq的学习在势在必行啊。今天总结下平时比较常用的表连接的用法。

Inner Join

Linq：

   var list = (from c in customerDb.Order
                             join o in customerDb.OrderItem on c.OrderId equals o.OrderItemId
                             select new { c = c.OrderId }).FirstOrDefault();

生成的Sql

 SELECT TOP (1)
 [Extent1].[OrderId] AS [OrderId]
 FROM  [dbo].[Orders] AS [Extent1]
 INNER JOIN [dbo].[OrderItems] AS [Extent2] ON [Extent1].[OrderId] = [Extent2].[OrderItemId]

Left Join

Linq:

      var list = (from c in customerDb.Order
                             join o in customerDb.OrderItem on c.OrderId equals o.OrderItemId
                             into Grp
                             from grp in Grp.DefaultIfEmpty()
                             select new { c = c.OrderId, grp = grp.OrderItemId }).FirstOrDefault();

生成的Sql:

 SELECT TOP (1)
 [Extent1].[OrderId] AS [OrderId],
 [Extent2].[OrderItemId] AS [OrderItemId]
 FROM  [dbo].[Orders] AS [Extent1]
 LEFT OUTER JOIN [dbo].[OrderItems] AS [Extent2] ON [Extent1].[OrderId] = [Extent2].[OrderItemId]

Cross Join

Linq：

     var list = (from c in customerDb.Order
                             from o in customerDb.OrderItem
                             select new { c = c.OrderId,o=o.OrderItemId }).FirstOrDefault();

生成的Sql:

 SELECT TOP ()
 [Extent1].[OrderId] AS [OrderId],
 [Extent2].[OrderItemId] AS [OrderItemId]
 FROM  [dbo].[Orders] AS [Extent1]
 CROSS JOIN [dbo].[OrderItems] AS [Extent2]

总结：右连接其实就是换下两个表的顺序位置，

全连接就是把左外连接的结果拼接起来去重就行。

用顺ef后，可以加速开发效率，期待那一天早日到来，加油啊！！！