复杂Linq语句写法

从网上收藏的复杂Linq语句写法

1、case when：

原型：

sql原型：

SELECT ProductID, Name, Color,

CASE

WHEN Color = 'Red' THEN StandardCost

WHEN Color = 'Black' THEN StandardCost + 10

ELSE ListPrice

END Price

FROM SalesLT.Product

Linq代码：

Products.Select(P => new

{

ID = P.ProductID,

Name = P.Name,

Color = P.Color,

Price = (P.Color == "Red" ? P.StandardCost : (P.Color == "Black" ? P.StandardCost + 10 : P.ListPrice))

});

2、where中使用case when

linq代码：

Products

.Where(P => (P.Color == "Red" ? (P.StandardCost > 100) : (P.Color == "Black" ? P.ListPrice > 100 : P.ListPrice == P.StandardCost)))

.Select(P => new

{

ID = P.ProductID,

Name = P.Name,

Color = P.Color,

StandardCost = P.StandardCost,

ListPrice = P.ListPrice

});

sql原型：

SELECT ProductID, Name, Color, StandardCost, ListPrice

FROM SalesLT.Product

WHERE (

(CASE

WHEN Color = 'Red' THEN

(CASE

WHEN StandardCost > 100 THEN 1

WHEN NOT (StandardCost > 100) THEN 0

ELSE NULL

END)

WHEN Color = 'Black' THEN

(CASE

WHEN ListPrice > 100 THEN 1

WHEN NOT (ListPrice > 100) THEN 0

ELSE NULL

END)

ELSE

(CASE

WHEN ListPrice = StandardCost THEN 1

WHEN NOT (ListPrice = StandardCost) THEN 0

ELSE NULL

END)

END)) = 1

3、group by中使用case when

linq代码：

Products.GroupBy(P => P.StandardCost > 1000? P.Color : P.SellStartDate.ToString())

sql原型：

-- Region Parameters

DECLARE @p0 Decimal(8,4) = 1000

-- EndRegion

SELECT [t1].[value] AS [Key]

FROM (

SELECT

(CASE

WHEN [t0].[StandardCost] > @p0 THEN CONVERT(NVarChar(MAX),[t0].[Color])

ELSE CONVERT(NVarChar(MAX),[t0].[SellStartDate])

END) AS [value]

FROM [SalesLT].[Product] AS [t0]

) AS [t1]

GROUP BY [t1].[value]

4、单表的查询

var query = from tc in db.tbClass

where tc.ClassID == "1"

//查询表tbClass

select new {

ClassID=tc.ClassID,

ClassName=tc.ClassName

5、多表内连接查询

var query = from s in db.tbStudents

join c in db.tbClass on s.ClassID equals c.ClassID

where s.ClassID == 3

select new

{

ClassID = s.ClassID,

ClassName = c.ClassName,

Student = new

{

Name = s.Name,

StudentID = s.StudentID

}

};

6、外连接

var query = from s in db.tbStudents

join c in db.tbClass on s.ClassID equals c.ClassID into tbC

from tbCw in tbC.DefaultIfEmpty()

where s.ClassID == 3

select new

{

ClassID = s.ClassID,

ClassName = tbCw.ClassName,

Student = new

{

Name = s.Name,

StudentID = s.StudentID

}

};

7、多表关联

listUser = (from u in dbcontext.t_user

join g in dbcontext.t_grade on u.gradeID equals g.gradeID

join s in dbcontext.t_states on u.state equals s.statesID

join c in dbcontext.t_character on u.levelID equals c.levelID

select new userModel()

{

userID=u.userID,

userName=u.userName,

userGrade=g.userGrade,

userStates=s.userSates,

userLevel=c.userLevel,

totalMcoin=u.totalMcoin

}).ToList();

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版权声明：本文为CSDN博主「xjzdr」的原创文章，遵循 CC 4.0 BY-SA 版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/xjzdr/article/details/81179290