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一步一步学Linq to sql（四）：查询句法

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一步一步学Linq to sql（四）：查询句法

作者: lovecherry 来源: 博客园发布时间:
2008-09-26 22:59 阅读: 33431 次推荐: 0 原文链接 [收藏]

[1] 一步一步学Linq to sql（四）：查询句法

[2] 一步一步学Linq to sql（四）：查询句法

[3] 一步一步学Linq to sql（四）：查询句法

[4] 一步一步学Linq to sql（四）：查询句法

[5] 一步一步学Linq to sql（四）：查询句法

[6] 一步一步学Linq to sql（四）：查询句法

系列文章导航：

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.FirstNameascending

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 selectc.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") selectc).Union

(from c in ctx.Customers where c.ContactName.StartsWith("A") selectc).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")selectc).Concat

(from c in ctx.Customers where c.ContactName.StartsWith("A") selectc).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") selectc).Intersect

(from c in ctx.Customers where c.ContactName.StartsWith("A") selectc).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") selectc).Except

(from c in ctx.Customers where c.ContactName.StartsWith("A") selectc).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 whereo.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。