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系列文章導航：
一步一步學Linq to sql（二）：DataContext與實體
select
描述：查詢顧客的公司名、地址信息
查詢句法：
var 構建匿名類型1 = from c in ctx.Customers select new { 公司名 = c.CompanyName, 地址 = c.Address }; |
對應SQL：
SELECT [t0].[CompanyName], [t0].[Address] FROM [dbo].[Customers] AS [t0] |
描述：查詢職員的姓名和雇用年份
查詢句法：
var 構建匿名類型2 = from emp in ctx.Employees select new { 姓名 = emp.LastName + emp.FirstName, 雇用年 = emp.HireDate.Value.Year }; |
對應SQL：
SELECT [t0].[LastName] + [t0].[FirstName] AS [value], DATEPART(Year, [t0].[HireDate]) AS [value2] FROM [dbo].[Employees] AS [t0] |
描述：查詢顧客的ID以及聯系信息（職位和聯系人）
查詢句法：
var 構建匿名類型3 = from c in ctx.Customers select new { ID = c.CustomerID, 聯系信息 = new { 職位 = c.ContactTitle, 聯系人 = c.ContactName } }; |
對應SQL：
SELECT [t0].[CustomerID], [t0].[ContactTitle], [t0].[ContactName] FROM [dbo].[Customers] AS [t0] |
描述：查詢訂單號和訂單是否超重的信息
查詢句法：
var select帶條件 = from o in ctx.Orders select new { 訂單號 = o.OrderID, 是否超重 = o.Freight > 100 ? "是" : "否" }; |
對應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] |
一步一步學Linq to sql（二）：DataContext與實體系列文章導航：
where
描述：查詢顧客的國家、城市和訂單數信息，要求國家是法國并且訂單數大于5
查詢句法：
var 多條件 = from c in ctx.Customers where c.Country == "France" && c.Orders.Count > 5 select new { 國家 = c.Country, 城市 = c.City, 訂單數 = c.Orders.Count }; |
對應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：查詢所有沒有下屬雇員的雇用年和名，按照雇用年倒序，按照名正序
查詢句法：
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 }; |
對應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] |
一步一步學Linq to sql（二）：DataContext與實體系列文章導航：
分頁
描述：按照每頁10條記錄，查詢第二頁的顧客
查詢句法：
var 分頁 = (from c in ctx.Customers select c).Skip(10).Take(10); |
對應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] |
分組
描述：根據顧客的國家分組，查詢顧客數大于5的國家名和顧客數
查詢句法：
var 一般分組 = from c in ctx.Customers group c by c.Country into g where g.Count() > 5 orderby g.Count() descending select new { 國家 = g.Key, 顧客數 = g.Count() }; |
對應SQL：
SELECT [t1].[Country], [t1].[value3] AS [顧客數] 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：根據國家和城市分組，查詢顧客覆蓋的國家和城市
查詢句法：
var 匿名類型分組 = from c in ctx.Customers group c by new { c.City, c.Country } into g orderby g.Key.Country, g.Key.City select new { 國家 = g.Key.Country, 城市 = g.Key.City }; |
對應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：按照是否超重條件分組，分別查詢訂單數量
查詢句法：
var 按照條件分組 = from o in ctx.Orders group o by new { 條件 = o.Freight > 100 } into g select new { 數量 = g.Count(), 是否超重 = g.Key.條件 ? "是" : "否" }; |
對應SQL：
SELECT (CASE WHEN [t2].[value2] = 1 THEN @p1 ELSE @p2 END) AS [value], [t2].[value] AS [數量] 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] |
一步一步學Linq to sql（二）：DataContext與實體系列文章導航：
distinct
描述：查詢顧客覆蓋的國家
查詢句法：
var 過濾相同項 = (from c in ctx.Customers orderby c.Country select c.Country).Distinct(); |
對應SQL：
SELECT DISTINCT [t0].[Country] FROM [dbo].[Customers] AS [t0] |
union
描述：查詢城市是A打頭和城市包含A的顧客并按照顧客名字排序
查詢句法：
var 連接并且過濾相同項 = (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); |
對應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：查詢城市是A打頭和城市包含A的顧客并按照顧客名字排序，相同的顧客信息不會過濾
查詢句法：
var 連接并且不過濾相同項 = (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); |
對應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%] |
一步一步學Linq to sql（二）：DataContext與實體系列文章導航：
取相交項
描述：查詢城市是A打頭的顧客和城市包含A的顧客的交集，并按照顧客名字排序
查詢句法：
var 取相交項 = (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); |
對應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%] |
排除相交項
描述：查詢城市包含A的顧客并從中刪除城市以A開頭的顧客，并按照顧客名字排序
查詢句法：
var 排除相交項 = (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); |
對應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%] |
子查詢
描述：查詢訂單數超過5的顧客信息
查詢句法：
var 子查詢 = 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; |
對應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] |
一步一步學Linq to sql（二）：DataContext與實體系列文章導航：
in操作
描述：查詢指定城市中的客戶
查詢句法：
var in操作 = from c in ctx.Customers where new string[] { "Brandenburg", "Cowes", "Stavern" }.Contains(c.City) select c; |
對應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：內連接，沒有分類的產品查詢不到
查詢句法：
var innerjoin = from p in ctx.Products join c in ctx.Categories on p.CategoryID equals c.CategoryID select p.ProductName; |
對應SQL：
SELECT COUNT(*) AS [value] FROM [dbo].[Products] AS [t0] INNER JOIN [dbo].[Categories] AS [t1] ON [t0].[CategoryID] = ([t1].[CategoryID]) |
描述：外連接，沒有分類的產品也能查詢到
查詢句法：
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; |
對應SQL：
SELECT COUNT(*) AS [value] FROM [dbo].[Products] AS [t0] LEFT OUTER JOIN [dbo].[Categories] AS [t1] ON [t0].[CategoryID] = ([t1].[CategoryID]) |
你可能會很奇怪，原先很復雜的SQL使用查詢句法會很簡單（比如按照條件分組）。但是原先覺得很好理解的SQL使用查詢句法會覺得很復雜（比如連接查詢）。其實，我們還可以通過其它方式進行連接操作，在以后說DataLoadOptions類型的時候會再說。雖然Linq to sql已經非常智能了，但是對于非常復雜的查詢還是建議通過存儲過程實現，下次講解如何調用存儲過程。
it知識庫：一步一步學Linq to sql（四）：查詢句法，轉載需保留來源！
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