一步一步学Linq to sql（四）：查询句法

时间：2021-07-03 20:35:44

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。

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