in left join inner join

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;

对应SQL：

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax] FROM [dbo].[Customers] AS [t0] WHERE [t0].[City] IN (@p0, @p1, @p2) -- @p0: Input String (Size = 11; Prec = 0; Scale = 0) [Brandenburg] -- @p1: Input String (Size = 5; Prec = 0; Scale = 0) [Cowes] -- @p2: Input String (Size = 7; Prec = 0; Scale = 0) [Stavern]

 

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;

对应SQL：

SELECT COUNT(*) AS [value] FROM [dbo].[Products] AS [t0] INNER JOIN [dbo].[Categories] AS [t1] ON [t0].[CategoryID] = ([t1].[CategoryID])

 

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;

对应SQL：

SELECT COUNT(*) AS [value] FROM [dbo].[Products] AS [t0] LEFT OUTER JOIN [dbo].[Categories] AS [t1] ON [t0].[CategoryID] = ([t1].[CategoryID])

 

       你可能会很奇怪，原先很复杂的SQL使用查询句法会很简单（比如按照条件分组）。但是原先觉得很好理解的SQL使用查询句法会觉得很复杂（比如连接查询）。其实，我们还可以通过其它方式进行连接操作，在以后说DataLoadOptions类型的时候会再说。虽然Linq to sql已经非常智能了，但是对于非常复杂的查询还是建议通过存储过程实现，下次讲解如何调用存储过程。