一、SQL、Linq、lamda表达式 同一功能不同写法

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)
   )

二、同一功能的几种不同写法的实例

1、简单的函数计算（count，min，max，sum）

 
C# 代码   复制

            //1


            ////获取最大的rpId

            var ss = (from r in db.Am_recProScheme

                      select r).Max(p => p.rpId);

            ////获取最小的rpId

            var ss = (from r in db.Am_recProScheme

                      select r).Min(p => p.rpId);

            //获取结果集的总数

              var ss = (from r in db.Am_recProScheme

                     select r).Count();

            //获取rpId的和

            var ss = (from r in db.Am_recProScheme

                      select r).Sum(p => p.rpId);





            //2

            var ss1 = db.Am_recProScheme.Max(p=>p.rpId);

            var ss1 = db.Am_recProScheme.Min(p => p.rpId);

            var ss1 = db.Am_recProScheme.Count() ;

            var ss1 = db.Am_recProScheme.Sum(p => p.rpId);

            Response.Write(ss);



            //3

            string sssql = "select max(rpId) from Am_recProScheme";

            sssql = "select min(rpId) from Am_recProScheme";

            sssql = "select count(1) from Am_recProScheme";

            sssql = "select sum(rpId) from Am_recProScheme";

2、排序order by desc/asc

 
C# 代码   复制

          var ss = from r in db.Am_recProScheme


                     where r.rpId > 10


                     orderby r.rpId descending  //倒序


                     //  orderby r.rpId ascending   //正序


                     select r;


          //正序


           var ss1 = db.Am_recProScheme.OrderBy(p => p.rpId).Where(p => p.rpId > 10).ToList();


          //倒序


           var ss2 = db.Am_recProScheme.OrderByDescending(p => p.rpId).Where(p => p.rpId > 10).ToList();


          string sssql = "select * from Am_recProScheme where rpid>10 order by rpId [desc|asc]";

3、top

 
 
C# 代码   复制

          //1


          //如果取最后一个可以按倒叙排列再取值


           var ss = (from r in db.Am_recProScheme select r).FirstOrDefault();


          //2


          string sssql = "select top(1) * from Am_recProScheme";

4、跳过前面多少条数据取余下的数据

 
 
C# 代码   复制

             //1


           var ss = (from r in db.Am_recProScheme


                      orderby r.rpId descending


                      select r).Skip(10); //跳过前10条数据，取10条之后的所有数据


             //2


           var ss1 = db.Am_recProScheme.OrderByDescending(p => p.rpId).Skip(10).ToList();


           //3


           string sssql = "select * from  (select ROW_NUMBER()over(order by rpId desc) as rowNum, * from [Am_recProScheme]) as t where rowNum>10";

5、分页数据查询

 
 
C# 代码   复制

             //1


           var ss = (from r in db.Am_recProScheme


                      where r.rpId > 10


                      orderby r.rpId descending


                      select r).Skip(10).Take(10); //取第11条到第20条数据


            //2 Take(10): 数据从开始获取，获取指定数量（10）的连续数据


            var ss1 = db.Am_recProScheme.OrderByDescending(p => p.rpId).Where(p => p.rpId > 10).Skip(10).Take(10).ToList();


           //3


           string sssql = "select * from  (select ROW_NUMBER()over(order by rpId desc) as rowNum, * from [Am_recProScheme]) as t where rowNum>10 and rowNum<=20";

6、包含，类似like '%%'

 
 
C# 代码   复制

              //1


            var ss = from r in db.Am_recProScheme


                     where r.SortsText.Contains("张")


                     select r;


            //2


            var ss1 = db.Am_recProScheme.Where(p => p.SortsText.Contains("张")).ToList();


            //3


            string sssql = "select * from Am_recProScheme where SortsText like '%张%'";

7、分组group by

 
 
C# 代码   复制

              //1


            var ss = from r in db.Am_recProScheme


                     orderby r.rpId descending


                     group r by r.recType into n


                     select new


                     {

                         n.Key,  //这个Key是recType

                         rpId = n.Sum(r => r.rpId), //组内rpId之和

                         MaxRpId = n.Max(r => r.rpId),//组内最大rpId

                         MinRpId = n.Min(r => r.rpId), //组内最小rpId

                     };

            foreach (var t in ss)

            {

                Response.Write(t.Key + "--" + t.rpId + "--" + t.MaxRpId + "--" + t.MinRpId);

            }



            //2

            var ss1 = from r in db.Am_recProScheme

                     orderby r.rpId descending

                     group r by r.recType into n

                     select n;

            foreach (var t in ss1)

            {

                Response.Write(t.Key + "--" + t.Min(p => p.rpId));

            }



            //3

            var ss2 = db.Am_recProScheme.GroupBy(p => p.recType);

            foreach (var t in ss2)

            {

                Response.Write(t.Key + "--" + t.Min(p => p.rpId));

            }



            //4

            string sssql = "select recType,min(rpId),max(rpId),sum(rpId) from Am_recProScheme group by recType";

8、连接查询 

 
 
C# 代码   复制

            //1


            var ss = from r in db.Am_recProScheme


                     join w in db.Am_Test_Result on r.rpId equals w.rsId


                     orderby r.rpId descending


                     select r;


            //2


            var ss1 = db.Am_recProScheme.Join(db.Am_Test_Result, p => p.rpId, r => r.rsId, (p, r) => p).OrderByDescending(p => p.rpId).ToList();


            //3


            string sssql = "select r.* from  [Am_recProScheme] as r inner join [dbo].[Am_Test_Result] as t on r.[rpId] = t.[rsId] order by r.[rpId] desc";

9、sql中的In

 
 
C# 代码   复制

              //1


            var ss = from p in db.Am_recProScheme


                              where (new int?[] { 24, 25,26 }).Contains(p.rpId)

                              select p;

            foreach (var p in ss)

            {

                Response.Write(p.Sorts);

            }



            //2

            string st = "select * from Am_recProScheme where rpId in(24,25,26)";

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