本文导读：LINQ是Language Integrated Query的简称，Linq包括Linq to Objects, Linq to SQL,Linq to XML, Linq to DataSet等，Lambda 表达式是一种可用于创建委托或表达式目录树类型的匿名函数。下面介绍Linq与lamda表达式相应的对比
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.LoginID, e.JobTitle FROM HumanResources.Employee AS e |
from e in Employees select new {e.LoginID, e.JobTitle} |
Employees |
|
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 DISTINCT e.JobTitle FROM HumanResources.Employee AS e |
(from e in Employees select e.JobTitle).Distinct() |
Employees |
|
SELECT e.* FROM HumanResources.Employee AS e WHERE e.LoginID = 'test' |
from e in Employees where e.LoginID == "test" select e |
Employees |
|
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 |
|
SELECT e.* WHERE e.VacationHours >= 2 AND e.VacationHours <= 10 |
from e in Employees where e.VacationHours >= 2 && e.VacationHours <= 10 select e |
Employees |
|
SELECT e.* FROM HumanResources.Employee AS e |
from e in Employees orderby e.NationalIDNumber select e |
Employees |
|
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 |
|
SELECT 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 |
|
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 |
|
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 |
|
SELECT * FROM Production.Product AS p, Production.ProductReview AS pr |
from p in Products from pr in ProductReviews select new {p, pr} |
Products |
|
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 |
|
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 |
|
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 |
|
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 TOP (10) * FROM Production.Product AS p WHERE p.StandardCost < 100 |
(from p in Products where p.StandardCost < 100 select p).Take(10) |
Products |
|
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
|
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）
//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
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
//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
//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
//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 '%%'
//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
//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
//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
//1
var ss = from p in db.Am_recProScheme
where (newint?[] { 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)";
转自：http://www.studyofnet.com/news/1095.html