SQL/LINQ/Lambda 转换

一、lamda 简单说就是个语法糖。

         本来你要声明实现一个方法，然后在调用，要写一大堆代码，可是微软总是认为程序员是懒散的，

如二楼的例子：

Func<int,int,int> fc=(int x,int y)=>x*y;

你看就一句代码，声明一个方法，返回值为int，参数是两个int。功能是计算两个整数的乘积。

二、LINQ是一种用来进行数据访问的编程模型，使得.NET语言可以直接支持数据查询。

–LINQ to Objects  用于对象的查询

–LINQ to XML  对XML数据的查询

–LINQ to ADO.NET  对数据库的查询

     •LINQ to DataSets  数据集

     •LINQ to Entities  ORM对象

     •LINQ to SQL  简易ORM框架

实现 IEnumerable<T> 或 IQueryable<T> 接口的对象都可使用LINQ操作。

三、SQL / LINQ / Lambda 对应列表

SQL	LINQ	Lambda
FROM
SELECT * FROM Employees	from e in Employees select e	Employees    .Select (e => e)
SELECT e.LoginID, e.JobTitle FROM Employees AS e	from e in Employees select new {e.LoginID, e.JobTitle}	Employees    .Select (      e =>          new          {             LoginID = e.LoginID,             JobTitle = e.JobTitle          }    )
SELECT e.LoginID AS ID, e.JobTitle AS Title FROM Employees AS e	from e in Employees select new {ID = e.LoginID, Title = e.JobTitle}	Employees    .Select (      e =>          new          {             ID = e.LoginID,             Title = e.JobTitle          }    )
DISTINCT 用来过滤掉多余的重复记录只保留一条
SELECT DISTINCT e.JobTitle FROM Employee AS e	(from e in Employees select e.JobTitle).Distinct()	Employees    .Select (e => e.JobTitle)    .Distinct ()
WHERE
SELECT e.* FROM .Employee AS e WHERE e.LoginID = 'test'	from e in Employees where e.LoginID == "test" select e	Employees    .Where (e => (e.LoginID == "test"))
SELECT e.* FROM Employee AS e WHERE e.LoginID = 'test' AND e.SalariedFlag = 1	from e in Employees where e.LoginID == "test"&& e.SalariedFlag select e	Employees    .Where (e => ((e.LoginID == "test") && e.SalariedFlag))
SELECT e.* FROM Employee AS e WHERE e.VacationHours> = 2 AND e.VacationHours <= 10	from e in Employees where e.VacationHours> = 2 && e.VacationHours <= 10 select e	Employees    .Where (e => (((Int32)(e.VacationHours) >= 2)&& ((Int32)(e.VacationHours) < = 10)))
ORDER BY
SELECT e.* FROM Employee AS e ORDER BY e.NationalIDNumber	from e in Employees orderby e.NationalIDNumber select e	Employees    .OrderBy (e => e.NationalIDNumber)
SELECT e.* FROM Employee AS e ORDER BY e.HireDate DESC, e.NationalIDNumber	from e in Employees orderby e.HireDate descending, e.NationalIDNumber select e	Employees   .OrderByDescending (e => e.HireDate)    .ThenBy (e => e.NationalIDNumber)
LIKE
SELECT e.* FROM Employee AS e WHERE e.JobTitle LIKE 'Vice%' OR SUBSTRING(e.JobTitle, 0, 3) = 'Pro'	from e in Employees where e.JobTitle.StartsWith("Vice") || e.JobTitle.Substring(0, 3) == "Pro" select e	Employees    .Where (e => (e.JobTitle.StartsWith ("Vice") || (e.JobTitle.Substring (0, 3) == "Pro")))
SUM
SELECT SUM(e.VacationHours) FROM Employee AS e		Employees.Sum(e => e.VacationHours);
COUNT
SELECT COUNT(*) FROM HumanResources.Employee AS e		Employees.Count();
SELECT SUM(e.VacationHours) AS TotalVacations, e.JobTitle FROM Employee AS e GROUP BY e.JobTitle	from e in Employees group e by e.JobTitle into g select new {JobTitle = g.Key, TotalVacations = g.Sum(e => e.VacationHours)}	Employees    .GroupBy (e => e.JobTitle)    .Select (      g =>          new          {             JobTitle = g.Key,             TotalVacations = g.Sum (e => (Int32)(e.VacationHours))          }    )
SELECT e.JobTitle, SUM(e.VacationHours) AS TotalVacations FROM Employee AS e GROUP BY e.JobTitle HAVING e.COUNT(*)> 2	from e in Employees group e by e.JobTitle into g where g.Count() > 2 select new {JobTitle = g.Key, TotalVacations = g.Sum(e => e.VacationHours)}	Employees    .GroupBy (e => e.JobTitle)    .Where (g => (g.Count () > 2))    .Select (      g =>          new          {             JobTitle = g.Key,             TotalVacations = g.Sum (e => (Int32)(e.VacationHours))          }    )
多表查询
SELECT * FROM Product AS p, ProductReview AS pr	from p in Products from pr in ProductReviews select new {p, pr}	Products    .SelectMany (      p => ProductReviews,      (p, pr) =>          new          {             p = p,             pr = pr          }    )
SELECT * FROM Product AS p INNER JOIN ProductReview AS pr ON p.ProductID = pr.ProductID	from p in Products join pr in ProductReviews on p.ProductID equals pr.ProductID select new {p, pr}	Products    .Join (       ProductReviews,      p => p.ProductID,      pr => pr.ProductID,      (p, pr) =>          new          {             p = p,             pr = pr          }    )
SELECT * FROM Product AS p INNER JOIN ProductCostHistory AS pch ON p.ProductID = pch.ProductID AND p.SellStartDate = pch.StartDate	from p in Products join pch in ProductCostHistories on new {p.ProductID, StartDate = p.SellStartDate} equals new {pch.ProductID, StartDate = pch.StartDate} select new {p, pch}	Products    .Join (       ProductCostHistories,      p =>          new          {             ProductID = p.ProductID,             StartDate = p.SellStartDate          },       pch =>          new          {             ProductID = pch.ProductID,             StartDate = pch.StartDate          },       (p, pch) =>          new          {             p = p,             pch = pch          }    )
SELECT * FROM Product AS p LEFT OUTER JOIN ProductReview AS pr ON p.ProductID = pr.ProductID	from p in Products join pr in ProductReviews on p.ProductID equals pr.ProductID into prodrev select new {p, prodrev}	Products    .GroupJoin (       ProductReviews,      p => p.ProductID,      pr => pr.ProductID,      (p, prodrev) =>          new          {             p = p,             prodrev = prodrev          }    )
SELECT p.ProductID AS ID FROM Product AS p UNION SELECT pr.ProductReviewID FROM ProductReview AS pr	(from p in Products select new {ID = p.ProductID}).Union( from pr in ProductReviews select new {ID = pr.ProductReviewID})	Products    .Select (      p =>          new          {             ID = p.ProductID          }    )    .Union (       ProductReviews          .Select (             pr =>                new                {                   ID = pr.ProductReviewID                }          )    )
SELECT TOP (10) * FROM Product AS p WHERE p.StandardCost< 100	(from p in Products where p.StandardCost< 100 select p).Take(10)	Products    .Where (p => (p.StandardCost < 100))    .Take (10)
SELECT * FROM [Production].[Product] AS p WHERE p.ProductID IN(    SELECT pr.ProductID    FROM [Production].[ProductReview] AS [pr]    WHERE pr.[Rating] = 5     )	from p in Products where (from pr in ProductReviews where pr.Rating == 5 select pr.ProductID).Contains(p.ProductID) select p	Products    .Where (      p =>          ProductReviews             .Where (pr => (pr.Rating == 5))             .Select (pr => pr.ProductID)             .Contains (p.ProductID)    )