转：Linq to SQL 经典查询 记录

SELECT
描述：查询顾客的公司名、地址信息

查询句法：

var 构建匿名类型1 = from c in ctx.Customers                       select new                       {                           公司名 = c.CompanyName,                           地址 = c.Address                       };

 

描述：查询职员的姓名和雇用年份

查询句法：

var 构建匿名类型2 = from emp in ctx.Employees                       select new                       {                           姓名 = emp.LastName + emp.FirstName,                           雇用年 = emp.HireDate.Value.Year                       };

 

描述：查询顾客的ID以及联系信息（职位和联系人）

查询句法：

var 构建匿名类型3 = from c in ctx.Customers                       select new                       {                           ID = c.CustomerID,                           联系信息 = new                           {                               职位 = c.ContactTitle,                               联系人 = c.ContactName                           }                       };

描述：查询订单号和订单是否超重的信息

查询句法：

var select带条件 = from o in ctx.Orders                         select new                         {                             订单号 = o.OrderID,                             是否超重 = o.Freight > 100 ? "是" : "否"                         };

 

WHERE

描述：查询顾客的国家、城市和订单数信息，要求国家是法国并且订单数大于5

查询句法：

var 多条件 = from c in ctx.Customers                   where c.Country == "France" && c.Orders.Count > 5                   select new                   {                       国家 = c.Country,                       城市 = c.City,                       订单数 = c.Orders.Count                   };

ORDER BY

描述：查询所有没有下属雇员的雇用年和名，按照雇用年倒序，按照名正序

查询句法：

var 排序 = from emp in ctx.Employees                  where emp.Employees.Count == 0                  orderby emp.HireDate.Value.Year descending, emp.FirstName ascending                  select new                  {                      雇用年 = emp.HireDate.Value.Year,                      名 = emp.FirstName                  };

GROUP BY

描述：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

查询句法：

var 一般分组 = from c in ctx.Customers                    group c by c.Country into g                    where g.Count() > 5                    orderby g.Count() descending                    select new                    {                        国家 = g.Key,                        顾客数 = g.Count()                    };

 

描述：根据国家和城市分组，查询顾客覆盖的国家和城市

查询句法：

var 匿名类型分组 = from c in ctx.Customers                      group c by new { c.City, c.Country } into g                      orderby g.Key.Country, g.Key.City                      select new                      {                          国家 = g.Key.Country,                          城市 = g.Key.City                      };

 

描述：按照是否超重条件分组，分别查询订单数量

查询句法：

var 按照条件分组 = from o in ctx.Orders                      group o by new { 条件 = o.Freight > 100 } into g                      select new                      {                          数量 = g.Count(),                          是否超重 = g.Key.条件 ? "是" : "否"                      };

 

DISTINCT

描述：查询顾客覆盖的国家

查询句法：

var 过滤相同项 = (from c in ctx.Customers orderby c.Country select c.Country).Distinct();

UNION

描述：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

查询句法：

var 连接并且过滤相同项 = (from c in ctx.Customers where c.City.Contains("A") select c).Union             (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

CONCAT

描述：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

查询句法：

var 连接并且不过滤相同项 = (from c in ctx.Customers where c.City.Contains("A") select c).Concat             (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

取项相交
描述：查询城市是A打头的顾客和城市包含A的顾客的交集，并按照顾客名字排序

查询句法：

var 取相交项 = (from c in ctx.Customers where c.City.Contains("A") select c).Intersect             (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

排除相交项

描述：查询城市包含A的顾客并从中删除城市以A开头的顾客，并按照顾客名字排序

查询句法：

var 排除相交项 = (from c in ctx.Customers where c.City.Contains("A") select c).Except             (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

子查询

描述：查询订单数超过5的顾客信息

查询句法：

var 子查询 = from c in ctx.Customers                    where                        (from o in ctx.Orders group o by o.CustomerID into o where o.Count() > 5 select o.Key).Contains(c.CustomerID)                    select c;

in 操作

描述：查询指定城市中的客户

查询句法：

var in操作 = from c in ctx.Customers                     where new string[] { "Brandenburg", "Cowes", "Stavern" }.Contains(c.City)                     select c;

Join

描述：内连接，没有分类的产品查询不到

查询句法：

var innerjoin = from p in ctx.Products                         join c in ctx.Categories                         on p.CategoryID equals c.CategoryID                         select p.ProductName;

 

描述：外连接，没有分类的产品也能查询到

查询句法：

var leftjoin = from p in ctx.Products                        join c in ctx.Categories                        on p.CategoryID equals c.CategoryID                        into pro                        from x in pro.DefaultIfEmpty()                        select p.ProductName;