本文将SQL、LINQ、Lambda 三种语法作一个横向对照，便于加深理解。

SQL LINQ Lambda
SELECT * FROM HumanResources.Employee from e in Employees select e Employees .Select (e => e)
SELECT e.LoginID, e.JobTitle FROM HumanResources.Employee 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 HumanResources.Employee AS e from e in Employees select new {ID = e.LoginID, Title = e.JobTitle} Employees.Select ( e => new { ID = e.LoginID, Title = e.JobTitle } )
SELECT DISTINCT e.JobTitle FROM HumanResources.Employee AS e (from e in Employees select e.JobTitle).Distinct() Employees .Select (e => e.JobTitle) .Distinct ()
SELECT e.* FROM HumanResources.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 HumanResources.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 HumanResources.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)))
SELECT e.* FROM HumanResources.Employee AS e ORDER BY e.NationalIDNumber from e in Employees orderby e.NationalIDNumber select e Employees .OrderBy (e => e.NationalIDNumber)
SELECT e.* FROM HumanResources.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)
SELECT e.* FROM HumanResources.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")))
SELECT SUM(e.VacationHours) FROM HumanResources.Employee AS e Employees.Sum(e => e.VacationHours);
SELECT COUNT(*) FROM HumanResources.Employee AS e Employees.Count();
SELECT SUM(e.VacationHours) AS TotalVacations, e.JobTitle FROM HumanResources.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 HumanResources.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)) } )
SELECT * FROM Production.Product AS p, Production.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 Production.Product AS p INNER JOIN Production.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 Production.Product AS p INNER JOIN Production.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 Production.Product AS p LEFT OUTER JOIN Production.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 Production.Product AS p UNION SELECT pr.ProductReviewID FROM Production.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 Production.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) )