概要：
LinQ的查询语法。再次重申此系列是我的学习笔记，仅供个人复习，在第一片有来源链接。
内容：
Select：
1， 查询顾客公司名，地址
Var tb=from c inctx..Customers
Select new
{
公司名=c.CompanyName,
地址=c.Address
};
SELECT [t0].[CompanyName], [t0].[Address]
FROM [dbo].[Customers] AS [t0]
2， 查询雇员的姓名和雇佣年龄
Var tb=from c inctx..Emplpyees
Select new
{
姓名=c.LastName+c.FirstName,
雇佣年龄=c.HireDate.Value.Year
};
SELECT [t0].[LastName] + [t0].[FirstName] AS [value], DATEPART(Year,
[t0].[HireDate]) AS [value2] FROM [dbo].[Employees] AS [t0]
3， 查询顾客ID和联系信息（职位，联系人）
Var tb=from c inctx..Customers
Select new
{
ID=c.CustomerID,
联系信息=new
{
职位=c.ContactTitle,
联系人=c.ContactName
}
};
SELECT [t0].[CustomerID], [t0].[ContactTitle], [t0].[ContactName]
FROM [dbo].[Customers] AS [t0]
4,查询订单号和订单是否超重
Vartb=from c in ctx .Orders
Selectnew
{
订单号=c.OrderID,
是否超重=c.Freight>100?”是”:”否”
};
Where：
1， 查询顾客国家，城市，订单数信息，要求国家是法国，订单数大于5
Var tb=from c inctx .Customers
Wherec.Country==”
Select new
{
国家=c.Country,
城市=c.City,
订单数=c.Orders.Count
};
2， 查询所有没有下属雇员的雇佣年和名，按照雇佣年倒序，名正序
Var tb=from x inctx.Employees
Wherec.Employees.Count==0
Orderbyc.HireDate.Value.Year descending,c.FirstName ascending
Select new
{
雇佣年=c.HireDate.Value.Year,
名=c.FirstName
};
分页：
1， 按照每页10条记录，查询第二页的顾客
Var tb=(from cin ctx.Customers select c).Skip(10).Take(10);
分组：
1， 根据顾客的国家分组，查询顾客数大于5的国家名和顾客数
Var tb=from c inctx.Customers
Group c byc.Country into g
Whereg.Count()>5
Orderbyg.Count() descending
Select new
{
国家=g.Key,
顾客数=g.Count()
};
2， 按照是否超重条件分组，分别查询订单数量
Var tb=from c in ctx .Orders
Group c by new {条件=c.Freight>100}into g
Select new
{
数量=g.Count(),
是否超重=g.Key.条件？”是”:”否”
};
Distinct:
1,查询顾客覆盖的国家
Var tb=(from c inctx.Customers orderby c.Country select c.Country).Distinct();
ELECT DISTINCT [t0].[Country]
FROM [dbo].[Customers] AS [t0]
1， 查询城市是A打头和城市包含A的顾客并按照顾客名字排序
Var 连接并过滤相同项=(fromc in ctx.Customers where c.City.Contains(“A”)
Select c).Union (from c in ctx.Customers wherec.ContactName.StartWith(“A”)
Select c).OrderBy(c=>c.ContactName);
Concat:
1, 查询城市是A打头和城市包含A的顾客并按照顾客名字排序，
相同的顾客信息不会过滤
var 连接并且不过滤相同项 = (fromc in ctx.Customers wherec.City.Contains("A")
select c).Concat(from c inctx.Customers where c.ContactName.StartsWith("A") select
c).OrderBy(c => c.ContactName);
子查询：
1,查询订单数超过5的顾客信息
Var tb=from c inctx.Customers
Where(from o in ctx.Ordersgroup o by o.CustomerID o where o.Count()>5
Selecto.Key).Contains(c.CustomerID)
Select c;
In操作：
1， 查询指定城市中的客户
Var in操作=from c inctx.Customers
Where new string[]{“
Select c;
Join连接：
1， 内连接，没有分类的产品查询不到
Var innerjoin=from p in ctx.Products
Join c in ctx.Categories
On p.CategoryID equals c.CategoryID
Select p.ProductName;
SELECT COUNT(*) AS [value]
FROM [dbo].[Products] AS [t0]
INNER JOIN [dbo].[Categories] AS [t1] ON [t0].[CategoryID] =([t1].[CategoryID])
2,外连接，没有分类的产品也能查询到
var leftjoin = from p in ctx.Products
join c inctx.Categories
on p.CategoryID equalsc.CategoryID
into pro
from x inpro.DefaultIfEmpty()
selectp.ProductName;
SELECT COUNT(*) AS [value]
FROM [dbo].[Products] AS [t0]
LEFT OUTER JOIN [dbo].[Categories] AS [t1] ON [t0].[CategoryID] =([t1].[CategoryID])