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： 

1   var list = (from c in customerDb.Order

2                             join o in customerDb.OrderItem on c.OrderId equals o.OrderItemId

3                             select new { c = c.OrderId }).FirstOrDefault();

      生成的Sql

1 SELECT TOP (1) 

2 [Extent1].[OrderId] AS [OrderId]

3 FROM  [dbo].[Orders] AS [Extent1]

4 INNER JOIN [dbo].[OrderItems] AS [Extent2] ON [Extent1].[OrderId] = [Extent2].[OrderItemId]

 

Left Join

      Linq:

1      var list = (from c in customerDb.Order

2                             join o in customerDb.OrderItem on c.OrderId equals o.OrderItemId

3                             into Grp

4                             from grp in Grp.DefaultIfEmpty()

5                             select new { c = c.OrderId, grp = grp.OrderItemId }).FirstOrDefault();

      生成的Sql:

1 SELECT TOP (1) 

2 [Extent1].[OrderId] AS [OrderId], 

3 [Extent2].[OrderItemId] AS [OrderItemId]

4 FROM  [dbo].[Orders] AS [Extent1]

5 LEFT OUTER JOIN [dbo].[OrderItems] AS [Extent2] ON [Extent1].[OrderId] = [Extent2].[OrderItemId]

 Cross Join

        Linq：   

1     var list = (from c in customerDb.Order

2                             from o in customerDb.OrderItem 

3                             select new { c = c.OrderId,o=o.OrderItemId }).FirstOrDefault();

        生成的Sql:

1 SELECT TOP (1) 

2 [Extent1].[OrderId] AS [OrderId], 

3 [Extent2].[OrderItemId] AS [OrderItemId]

4 FROM  [dbo].[Orders] AS [Extent1]

5 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！！！