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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原文链接：https://blog.csdn.net/xjzdr/article/details/81179290