一步一步学Linq to sql（四）：查询句法

select
描述：查询顾客的公司名、地址信息
查询句法：

var 构建匿名类型1 = from c in ctx.Customers select new { 公司名 = c.CompanyName, 地址 = c.Address };

对应SQL：

SELECT [t0].[CompanyName], [t0].[Address] FROM [dbo].[Customers] AS [t0]

描述：查询职员的姓名和雇用年份
查询句法：

var 构建匿名类型2 = from emp in ctx.Employees select new { 姓名 = emp.LastName + emp.FirstName, 雇用年 = emp.HireDate.Value.Year };

对应SQL：

SELECT [t0].[LastName] + [t0].[FirstName] AS [value], DATEPART(Year, [t0].[HireDate]) AS [value2] FROM [dbo].[Employees] AS [t0]

描述：查询顾客的ID以及联系信息（职位和联系人）
查询句法：

var 构建匿名类型3 = from c in ctx.Customers select new { ID = c.CustomerID, 联系信息 = new { 职位 = c.ContactTitle, 联系人 = c.ContactName } };

对应SQL：

SELECT [t0].[CustomerID], [t0].[ContactTitle], [t0].[ContactName] FROM [dbo].[Customers] AS [t0]

描述：查询订单号和订单是否超重的信息
查询句法：

var select带条件 = from o in ctx.Orders select new { 订单号 = o.OrderID, 是否超重 = o.Freight > 100 ? "是" : "否" };

对应SQL：

SELECT [t0].[OrderID], (CASE WHEN [t0].[Freight] > @p0 THEN @p1 ELSE @p2 END) AS [value] FROM [dbo].[Orders] AS [t0] -- @p0: Input Currency (Size = 0; Prec = 19; Scale = 4) [100] -- @p1: Input String (Size = 1; Prec = 0; Scale = 0) [是] -- @p2: Input String (Size = 1; Prec = 0; Scale = 0) [否]

where

描述：查询顾客的国家、城市和订单数信息，要求国家是法国并且订单数大于5
查询句法：

var 多条件 = from c in ctx.Customers where c.Country == "France" && c.Orders.Count > 5 select new { 国家 = c.Country, 城市 = c.City, 订单数 = c.Orders.Count };

对应SQL：

SELECT [t0].[Country], [t0].[City], ( SELECT COUNT(*) FROM [dbo].[Orders] AS [t2] WHERE [t2].[CustomerID] = [t0].[CustomerID] ) AS [value] FROM [dbo].[Customers] AS [t0] WHERE ([t0].[Country] = @p0) AND ((( SELECT COUNT(*) FROM [dbo].[Orders] AS [t1] WHERE [t1].[CustomerID] = [t0].[CustomerID] )) > @p1) -- @p0: Input String (Size = 6; Prec = 0; Scale = 0) [France] -- @p1: Input Int32 (Size = 0; Prec = 0; Scale = 0) [5]

orderby

描述：查询所有没有下属雇员的雇用年和名，按照雇用年倒序，按照名正序
查询句法：

var 排序 = from emp in ctx.Employees where emp.Employees.Count == 0 orderby emp.HireDate.Value.Year descending, emp.FirstName ascending select new { 雇用年 = emp.HireDate.Value.Year, 名 = emp.FirstName };

对应SQL：

SELECT DATEPART(Year, [t0].[HireDate]) AS [value], [t0].[FirstName] FROM [dbo].[Employees] AS [t0] WHERE (( SELECT COUNT(*) FROM [dbo].[Employees] AS [t1] WHERE [t1].[ReportsTo] = [t0].[EmployeeID] )) = @p0 ORDER BY DATEPART(Year, [t0].[HireDate]) DESC, [t0].[FirstName] -- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [0]

分页

描述：按照每页10条记录，查询第二页的顾客
查询句法：

var 分页 = (from c in ctx.Customers select c).Skip(10).Take(10);

对应SQL：

SELECT TOP 10 [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName], [t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode], [t1].[Country], [t1].[Phone], [t1].[Fax] FROM ( SELECT ROW_NUMBER() OVER (ORDER BY [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]) AS [ROW_NUMBER], [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] ) AS [t1] WHERE [t1].[ROW_NUMBER] > @p0 -- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [10]

分组

描述：根据顾客的国家分组，查询顾客数大于5的国家名和顾客数
查询句法：

var 一般分组 = from c in ctx.Customers group c by c.Country into g where g.Count() > 5 orderby g.Count() descending select new { 国家 = g.Key, 顾客数 = g.Count() };

对应SQL：

SELECT [t1].[Country], [t1].[value3] AS [顾客数] FROM ( SELECT COUNT(*) AS [value], COUNT(*) AS [value2], COUNT(*) AS [value3], [t0].[Country] FROM [dbo].[Customers] AS [t0] GROUP BY [t0].[Country] ) AS [t1] WHERE [t1].[value] > @p0 ORDER BY [t1].[value2] DESC -- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [5]

描述：根据国家和城市分组，查询顾客覆盖的国家和城市
查询句法：

var 匿名类型分组 = from c in ctx.Customers group c by new { c.City, c.Country } into g orderby g.Key.Country, g.Key.City select new { 国家 = g.Key.Country, 城市 = g.Key.City };

对应SQL：

SELECT [t1].[Country], [t1].[City] FROM ( SELECT [t0].[City], [t0].[Country] FROM [dbo].[Customers] AS [t0] GROUP BY [t0].[City], [t0].[Country] ) AS [t1] ORDER BY [t1].[Country], [t1].[City]

描述：按照是否超重条件分组，分别查询订单数量
查询句法：

var 按照条件分组 = from o in ctx.Orders group o by new { 条件 = o.Freight > 100 } into g select new { 数量 = g.Count(), 是否超重 = g.Key.条件 ? "是" : "否" };

对应SQL：

SELECT (CASE WHEN [t2].[value2] = 1 THEN @p1 ELSE @p2 END) AS [value], [t2].[value] AS [数量] FROM ( SELECT COUNT(*) AS [value], [t1].[value] AS [value2] FROM ( SELECT (CASE WHEN [t0].[Freight] > @p0 THEN 1 WHEN NOT ([t0].[Freight] > @p0) THEN 0 ELSE NULL END) AS [value] FROM [dbo].[Orders] AS [t0] ) AS [t1] GROUP BY [t1].[value] ) AS [t2] -- @p0: Input Currency (Size = 0; Prec = 19; Scale = 4) [100] -- @p1: Input String (Size = 1; Prec = 0; Scale = 0) [是] -- @p2: Input String (Size = 1; Prec = 0; Scale = 0) [否]

distinct

描述：查询顾客覆盖的国家
查询句法：

var 过滤相同项 = (from c in ctx.Customers orderby c.Country select c.Country).Distinct();

对应SQL：

SELECT DISTINCT [t0].[Country] FROM [dbo].[Customers] AS [t0]

union

描述：查询城市是A打头和城市包含A的顾客并按照顾客名字排序
查询句法：

var 连接并且过滤相同项 = (from c in ctx.Customers where c.City.Contains("A") select c).Union (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

对应SQL：

SELECT [t3].[CustomerID], [t3].[CompanyName], [t3].[ContactName], [t3].[ContactTitle], [t3].[Address], [t3].[City], [t3].[Region], [t3].[PostalCode], [t3].[Country], [t3].[Phone], [t3].[Fax] FROM ( SELECT [t2].[CustomerID], [t2].[CompanyName], [t2].[ContactName], [t2].[ContactTitle], [t2].[Address], [t2].[City], [t2].[Region], [t2].[PostalCode], [t2].[Country], [t2].[Phone], [t2].[Fax] FROM ( 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] LIKE @p0 UNION SELECT [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName], [t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode], [t1].[Country], [t1].[Phone], [t1].[Fax] FROM [dbo].[Customers] AS [t1] WHERE [t1].[ContactName] LIKE @p1 ) AS [t2] ) AS [t3] ORDER BY [t3].[ContactName] -- @p0: Input String (Size = 3; Prec = 0; Scale = 0) [%A%] -- @p1: Input String (Size = 2; Prec = 0; Scale = 0) [A%]

concat

描述：查询城市是A打头和城市包含A的顾客并按照顾客名字排序，相同的顾客信息不会过滤
查询句法：

var 连接并且不过滤相同项 = (from c in ctx.Customers where c.City.Contains("A") select c).Concat (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

对应SQL：

SELECT [t3].[CustomerID], [t3].[CompanyName], [t3].[ContactName], [t3].[ContactTitle], [t3].[Address], [t3].[City], [t3].[Region], [t3].[PostalCode], [t3].[Country], [t3].[Phone], [t3].[Fax] FROM ( SELECT [t2].[CustomerID], [t2].[CompanyName], [t2].[ContactName], [t2].[ContactTitle], [t2].[Address], [t2].[City], [t2].[Region], [t2].[PostalCode], [t2].[Country], [t2].[Phone], [t2].[Fax] FROM ( 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] LIKE @p0 UNION ALL SELECT [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName], [t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode], [t1].[Country], [t1].[Phone], [t1].[Fax] FROM [dbo].[Customers] AS [t1] WHERE [t1].[ContactName] LIKE @p1 ) AS [t2] ) AS [t3] ORDER BY [t3].[ContactName] -- @p0: Input String (Size = 3; Prec = 0; Scale = 0) [%A%] -- @p1: Input String (Size = 2; Prec = 0; Scale = 0) [A%]

取相交项

描述：查询城市是A打头的顾客和城市包含A的顾客的交集，并按照顾客名字排序
查询句法：

var 取相交项 = (from c in ctx.Customers where c.City.Contains("A") select c).Intersect (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

对应SQL：

SELECT [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName], [t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode], [t1].[Country], [t1].[Phone], [t1].[Fax] FROM ( SELECT DISTINCT [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] ) AS [t1] WHERE (EXISTS( SELECT NULL AS [EMPTY] FROM [dbo].[Customers] AS [t2] WHERE ([t1].[CustomerID] = [t2].[CustomerID]) AND ([t2].[ContactName] LIKE @p0) )) AND ([t1].[City] LIKE @p1) ORDER BY [t1].[ContactName] -- @p0: Input String (Size = 2; Prec = 0; Scale = 0) [A%] -- @p1: Input String (Size = 3; Prec = 0; Scale = 0) [%A%]

排除相交项

描述：查询城市包含A的顾客并从中删除城市以A开头的顾客，并按照顾客名字排序
查询句法：

var 排除相交项 = (from c in ctx.Customers where c.City.Contains("A") select c).Except (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

对应SQL：

SELECT [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName], [t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode], [t1].[Country], [t1].[Phone], [t1].[Fax] FROM ( SELECT DISTINCT [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] ) AS [t1] WHERE (NOT (EXISTS( SELECT NULL AS [EMPTY] FROM [dbo].[Customers] AS [t2] WHERE ([t1].[CustomerID] = [t2].[CustomerID]) AND ([t2].[ContactName] LIKE @p0) ))) AND ([t1].[City] LIKE @p1) ORDER BY [t1].[ContactName] -- @p0: Input String (Size = 2; Prec = 0; Scale = 0) [A%] -- @p1: Input String (Size = 3; Prec = 0; Scale = 0) [%A%]

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;

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 EXISTS( SELECT NULL AS [EMPTY] FROM ( SELECT COUNT(*) AS [value], [t1].[CustomerID] FROM [dbo].[Orders] AS [t1] GROUP BY [t1].[CustomerID] ) AS [t2] WHERE ([t2].[CustomerID] = [t0].[CustomerID]) AND ([t2].[value] > @p0) ) -- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [5]

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。