LINQ学习
// from s in d.StorageList
// from f in s.FreightRelationList
// where f.Departmetnid == departmentcode && d.Code == delivery
// select s).ToList();
//var list1 = (from s in _storageRepository.Query()
// .FetchMany(p => p.DeliveryList).FetchMany(p => p.FreightRelationList)
// from d in s.DeliveryList
// from f in s.FreightRelationList
// where f.Departmetnid == departmentcode && d.Code == delivery
// select s).ToList();
var list2 = _storageRepository.Query()
.FetchMany(p => p.DeliveryList).FetchMany(p => p.FreightRelationList)
.Where(p => p.DeliveryList.Any(a => a.Code == delivery) && p.FreightRelationList.Any(a => a.Departmetnid == departmentcode)).ToList();
return list2;
list,set的转换
List list = new ArrayList(set);
Set set = new HashSet(list);
List<t_ExpoEntity> t_ExpoEntityList = new List<t_ExpoEntity>();
dd.Author = "11";
t_ExpoEntityList.Add(dd);
dd = new t_ExpoEntity();
dd.Author = "22";
t_ExpoEntityList.Add(dd);
List<t_ExpoEntity> t_ExpoEntityList2 = t_ExpoEntityList.FindAll(
delegate(t_ExpoEntity temp)
{
return temp.Author == "11";
}
);
LINQ TO SQL中的selectMany
首先看SelectMany的定义：
Queryable中的SelectMany 方法:将序列的每个元素投影到一个 IEnumerable<(Of <(T>)>) 并将结果序列组合为一个 IQueryable<(Of <(T>)>) 类型的序列。(引用MSDN)
在用LINQ TO SQL 来写查询语句时,有一个selectMany的语句,它标示着一对多的关系,这篇文章我想说下在LINQ TO SQL中几种可以等同selectMany的用法。
系统转换成selectMany的条件:
1:语句中不包含join ,into;
2:需要2个以上的from:下面以两个表为例:如第一个表from c in 表1
1):如果from的对象均用表名,(from c in 表2),则会转换成cross join;
2):如果第二个表名以第一个表的子表形式出现,即类似c.表2,这又分两种情况,
1>:from o in c.表2,此时会形成inner join
2>:from p in c.表2.DefaultIfEmpty(),此时会形成LEFT OUT JOIN
文中例子表结构说明:Customer表和Purchase表，通过ID与CustomerID建立关联。
[ID] [int] NOT NULL,
[Name] [nvarchar](30) )
CREATE TABLE [dbo].[Purchase](
[ID] [int] NOT NULL,
[CustomerID] [int] NULL,
[Date] [datetime] NOT NULL,
[Description] [varchar](30) )
我们来实现SQL中的三种非常经典的联接方式。
第一:cross join,它的结果集是所有表的迪卡尔积。
from c in Customers
from o in Purchases
select o
在LINQ TO SQL中,下面的from都指定为表名的话,就会生成下面的语句:
FROM [Customer] AS [t0], [Purchase] AS [t1]
第二:inner join。
from c in Customers
from o in c.Purchases
select o
生成的SQL如下:
FROM [Customer] AS [t0], [Purchase] AS [t1]
WHERE [t1].[CustomerID] = [t0].[ID]
虽然没有显示的用inner join,但和它的功能是一样的.它的写法和上面的cross join看起来特别像,唯一的区别就在于cross join时,直接用了表名Purchases,而inner join用的时候变成了c.Pruchasex,即形成了一对多的情况。
第三: LEFT OUTER JOIN
from p in c.Purchases.DefaultIfEmpty()
select new { c.Name, p.Description, Price = (decimal?) p.Price }
生成的SQL如下:
FROM [Customer] AS [t0]
LEFT OUTER JOIN [Purchase] AS [t1] ON [t1].[CustomerID] = [t0].[ID]
left outer join实际上是在inner join的基础上加了一个条件,利用DefaultIfEmpty(),当记录不匹配时,返回null
我们对上在的查询增加一个过滤条件。
from p in c.Purchases.Where (p => p.Price > 1000).DefaultIfEmpty()
select new
{
c.Name,
p.Description,
Price = (decimal?) p.Price
}
对应的SQL：
FROM [Customer] AS [t0]
LEFT OUTER JOIN [Purchase] AS [t1] ON ([t1].[Price] > @p0) AND ([t1].[CustomerID] = [t0].[ID])
此时上面的语句还是标准的LEFT OUT JOIN,如果我们改变下条件的位置呢?
from p in c.Purchases.DefaultIfEmpty()
where p.Price>1000
select new
{
c.Name,
p.Description,
Price = (decimal?) p.Price
}
对应的SQL：
FROM [Customer] AS [t0]
LEFT OUTER JOIN [Purchase] AS [t1] ON [t1].[CustomerID] = [t0].[ID]
WHERE [t1].[Price] > @p0
条件改变位置后并没有改变join的本质,还是LEFT OUT JOIN,只不过查询的结果不一样了,从结果集上看,后面一种的效果和inner join的结果一样。
总结：上面的查询语句也可以用显示的join来查询，个人更喜欢用显示的join,因为相比较SQL更接近些，看起来要亲近些。在下篇文章中，我会总结显示用join查询的用法，其实最终的显示结果都一样，只是写法不同。
List<Teacher> teachers = new List<Teacher>
{
new Teacher("a",new List<Student>{ new Student(100),new Student(90),new Student(30) }),
new Teacher("b",new List<Student>{ new Student(100),new Student(90),new Student(60) }),
new Teacher("c",new List<Student>{ new Student(100),new Student(90),new Student(40) }),
new Teacher("d",new List<Student>{ new Student(100),new Student(90),new Student(60) }),
new Teacher("e",new List<Student>{ new Student(100),new Student(90),new Student(50) }),
new Teacher("f",new List<Student>{ new Student(100),new Student(90),new Student(60) }),
new Teacher("g",new List<Student>{ new Student(100),new Student(90),new Student(60) })
};
var list1 = from t in teachers
from s in t.Students
where s.Score < 60
select s;
var list2 = teachers.SelectMany(t => t.Students).Where(s => s.Score < 60);
var list3 = teachers.SelectMany(
t => t.Students,
(t, s) => new { t.Name, s.Score })
.Where(n => n.Score < 60);
