一步一步学Linq to sql（四）：查询句法

2023-11-22 07:08
文章标签 sql 查询 一步 database linq 句法

本文主要是介绍一步一步学Linq to sql（四）：查询句法，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

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

                      };

对应SQL：

SELECT [t0].[CompanyName], [t0].[Address]

FROM [dbo].[Customers] AS [t0]      

 

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

                      };

对应SQL：

SELECT [t0].[LastName] + [t0].[FirstName] AS [value], DATEPART(Year, [t0].[HireDate]) AS [value2]

FROM [dbo].[Employees] AS [t0]

 

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

                          }

                      };

对应SQL：

SELECT [t0].[CustomerID], [t0].[ContactTitle], [t0].[ContactName]

FROM [dbo].[Customers] AS [t0]

 

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"

                        };

对应SQL：

SELECT [t0].[OrderID],

    (CASE

        WHEN [t0].[Freight] > @p0 THEN @p1

        ELSE @p2

     END) AS [value]

FROM [dbo].[Orders] AS [t0]

-- @p0: Input Currency (Size = 0; Prec = 19; Scale = 4) [100]

-- @p1: Input String (Size = 1; Prec = 0; Scale = 0) [是]

-- @p2: Input String (Size = 1; Prec = 0; Scale = 0) [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

                  };      

对应SQL：

SELECT [t0].[Country], [t0].[City], (

    SELECT COUNT(*)

    FROM [dbo].[Orders] AS [t2]

    WHERE [t2].[CustomerID] = [t0].[CustomerID]

    ) AS [value]

FROM [dbo].[Customers] AS [t0]

WHERE ([t0].[Country] = @p0) AND (((

    SELECT COUNT(*)

    FROM [dbo].[Orders] AS [t1]

    WHERE [t1].[CustomerID] = [t0].[CustomerID]

    )) > @p1)

-- @p0: Input String (Size = 6; Prec = 0; Scale = 0) [France]

-- @p1: Input Int32 (Size = 0; Prec = 0; Scale = 0) [5]    

 

orderby

 

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

                 };     

对应SQL：

SELECT DATEPART(Year, [t0].[HireDate]) AS [value], [t0].[FirstName]

FROM [dbo].[Employees] AS [t0]

WHERE ((

    SELECT COUNT(*)

    FROM [dbo].[Employees] AS [t1]

    WHERE [t1].[ReportsTo] = [t0].[EmployeeID]

    )) = @p0

ORDER BY DATEPART(Year, [t0].[HireDate]) DESC, [t0].[FirstName]

-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [0]

 

分页

 

描述：按照每页10条记录，查询第二页的顾客

查询句法：

  var 分页 = (from c in ctx.Customers select c).Skip(10).Take(10);

对应SQL：

SELECT TOP 10 [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName], [t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode], [t1].[Country], [t1].[Phone], [t1].[Fax]

FROM (

    SELECT ROW_NUMBER() OVER (ORDER BY [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]) AS [ROW_NUMBER], [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]

    FROM [dbo].[Customers] AS [t0]

    ) AS [t1]

WHERE [t1].[ROW_NUMBER] > @p0

-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [10]

 

分组

 

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()

                   };

对应SQL：

SELECT [t1].[Country], [t1].[value3] AS [顾客数]

FROM (

    SELECT COUNT(*) AS [value], COUNT(*) AS [value2], COUNT(*) AS [value3], [t0].[Country]

    FROM [dbo].[Customers] AS [t0]

    GROUP BY [t0].[Country]

    ) AS [t1]

WHERE [t1].[value] > @p0

ORDER BY [t1].[value2] DESC

-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [5]

 

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

                     };

对应SQL：

SELECT [t1].[Country], [t1].[City]

FROM (

    SELECT [t0].[City], [t0].[Country]

    FROM [dbo].[Customers] AS [t0]

    GROUP BY [t0].[City], [t0].[Country]

    ) AS [t1]

ORDER BY [t1].[Country], [t1].[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"

                     };

对应SQL：

SELECT

    (CASE

        WHEN [t2].[value2] = 1 THEN @p1

        ELSE @p2

     END) AS [value], [t2].[value] AS [数量]

FROM (

    SELECT COUNT(*) AS [value], [t1].[value] AS [value2]

    FROM (

        SELECT

            (CASE

                WHEN [t0].[Freight] > @p0 THEN 1

                WHEN NOT ([t0].[Freight] > @p0) THEN 0

                ELSE NULL

             END) AS [value]

        FROM [dbo].[Orders] AS [t0]

        ) AS [t1]

    GROUP BY [t1].[value]

    ) AS [t2]

-- @p0: Input Currency (Size = 0; Prec = 19; Scale = 4) [100]

-- @p1: Input String (Size = 1; Prec = 0; Scale = 0) [是]

-- @p2: Input String (Size = 1; Prec = 0; Scale = 0) [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();

对应SQL：

SELECT DISTINCT [t0].[Country]

FROM [dbo].[Customers] AS [t0]

 

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);

对应SQL：

SELECT [t3].[CustomerID], [t3].[CompanyName], [t3].[ContactName], [t3].[ContactTitle], [t3].[Address], [t3].[City], [t3].[Region], [t3].[PostalCode], [t3].[Country], [t3].[Phone], [t3].[Fax]

FROM (

    SELECT [t2].[CustomerID], [t2].[CompanyName], [t2].[ContactName], [t2].[ContactTitle], [t2].[Address], [t2].[City], [t2].[Region], [t2].[PostalCode], [t2].[Country], [t2].[Phone], [t2].[Fax]

    FROM (

        SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]

        FROM [dbo].[Customers] AS [t0]

        WHERE [t0].[City] LIKE @p0

        UNION

        SELECT [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName], [t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode], [t1].[Country], [t1].[Phone], [t1].[Fax]

        FROM [dbo].[Customers] AS [t1]

        WHERE [t1].[ContactName] LIKE @p1

        ) AS [t2]

    ) AS [t3]

ORDER BY [t3].[ContactName]

-- @p0: Input String (Size = 3; Prec = 0; Scale = 0) [%A%]

-- @p1: Input String (Size = 2; Prec = 0; Scale = 0) [A%]

 

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);

对应SQL：

SELECT [t3].[CustomerID], [t3].[CompanyName], [t3].[ContactName], [t3].[ContactTitle], [t3].[Address], [t3].[City], [t3].[Region], [t3].[PostalCode], [t3].[Country], [t3].[Phone], [t3].[Fax]

FROM (

    SELECT [t2].[CustomerID], [t2].[CompanyName], [t2].[ContactName], [t2].[ContactTitle], [t2].[Address], [t2].[City], [t2].[Region], [t2].[PostalCode], [t2].[Country], [t2].[Phone], [t2].[Fax]

    FROM (

        SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]

        FROM [dbo].[Customers] AS [t0]

        WHERE [t0].[City] LIKE @p0

        UNION ALL

        SELECT [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName], [t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode], [t1].[Country], [t1].[Phone], [t1].[Fax]

        FROM [dbo].[Customers] AS [t1]

        WHERE [t1].[ContactName] LIKE @p1

        ) AS [t2]

    ) AS [t3]

ORDER BY [t3].[ContactName]

-- @p0: Input String (Size = 3; Prec = 0; Scale = 0) [%A%]

-- @p1: Input String (Size = 2; Prec = 0; Scale = 0) [A%]

 

取相交项

 

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);

对应SQL：

SELECT [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName], [t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode], [t1].[Country], [t1].[Phone], [t1].[Fax]

FROM (

    SELECT DISTINCT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]

    FROM [dbo].[Customers] AS [t0]

    ) AS [t1]

WHERE (EXISTS(

    SELECT NULL AS [EMPTY]

    FROM [dbo].[Customers] AS [t2]

    WHERE ([t1].[CustomerID] = [t2].[CustomerID]) AND ([t2].[ContactName] LIKE @p0)

    )) AND ([t1].[City] LIKE @p1)

ORDER BY [t1].[ContactName]

-- @p0: Input String (Size = 2; Prec = 0; Scale = 0) [A%]

-- @p1: Input String (Size = 3; Prec = 0; Scale = 0) [%A%]

 

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);

对应SQL：

SELECT [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName], [t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode], [t1].[Country], [t1].[Phone], [t1].[Fax]

FROM (

    SELECT DISTINCT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]

    FROM [dbo].[Customers] AS [t0]

    ) AS [t1]

WHERE (NOT (EXISTS(

    SELECT NULL AS [EMPTY]

    FROM [dbo].[Customers] AS [t2]

    WHERE ([t1].[CustomerID] = [t2].[CustomerID]) AND ([t2].[ContactName] LIKE @p0)

    ))) AND ([t1].[City] LIKE @p1)

ORDER BY [t1].[ContactName]

-- @p0: Input String (Size = 2; Prec = 0; Scale = 0) [A%]

-- @p1: Input String (Size = 3; Prec = 0; Scale = 0) [%A%]

 

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;

对应SQL：

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]

FROM [dbo].[Customers] AS [t0]

WHERE EXISTS(

    SELECT NULL AS [EMPTY]

    FROM (

        SELECT COUNT(*) AS [value], [t1].[CustomerID]

        FROM [dbo].[Orders] AS [t1]

        GROUP BY [t1].[CustomerID]

        ) AS [t2]

    WHERE ([t2].[CustomerID] = [t0].[CustomerID]) AND ([t2].[value] > @p0)

    )

-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [5]

 

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;

对应SQL：

       SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]

FROM [dbo].[Customers] AS [t0]

WHERE [t0].[City] IN (@p0, @p1, @p2)

-- @p0: Input String (Size = 11; Prec = 0; Scale = 0) [Brandenburg]

-- @p1: Input String (Size = 5; Prec = 0; Scale = 0) [Cowes]

-- @p2: Input String (Size = 7; Prec = 0; Scale = 0) [Stavern]

 

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;

对应SQL：

SELECT COUNT(*) AS [value]

FROM [dbo].[Products] AS [t0]

INNER JOIN [dbo].[Categories] AS [t1] ON [t0].[CategoryID] = ([t1].[CategoryID])

 

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;

对应SQL：

SELECT COUNT(*) AS [value]

FROM [dbo].[Products] AS [t0]

LEFT OUTER JOIN [dbo].[Categories] AS [t1] ON [t0].[CategoryID] = ([t1].[CategoryID])

 

       你可能会很奇怪，原先很复杂的SQL使用查询句法会很简单（比如按照条件分组）。但是原先觉得很好理解的SQL使用查询句法会觉得很复杂（比如连接查询）。其实，我们还可以通过其它方式进行连接操作，在以后说DataLoadOptions类型的时候会再说。虽然Linq to sql已经非常智能了，但是对于非常复杂的查询还是建议通过存储过程实现，下次讲解如何调用存储过程。

这篇关于一步一步学Linq to sql（四）：查询句法的文章就介绍到这儿，希望我们推荐的文章对编程师们有所帮助！


http://www.chinasem.cn/article/408392。

相关文章

MySQL数据库双机热备的配置方法详解

MySQL数据库双机热备的配置方法详解

《MySQL数据库双机热备的配置方法详解》在企业级应用中,数据库的高可用性和数据的安全性是至关重要的,MySQL作为最流行的开源关系型数据库管理系统之一,提供了多种方式来实现高可用性,其中双机热备（M... 目录1. 环境准备1.1 安装mysql1.2 配置MySQL1.2.1 主服务器配置1.2.2 从
阅读更多...
深入理解Mysql OnlineDDL的算法

深入理解Mysql OnlineDDL的算法

《深入理解MysqlOnlineDDL的算法》本文主要介绍了讲解MysqlOnlineDDL的算法,文中通过示例代码介绍的非常详细,对大家的学习或者工作具有一定的参考学习价值,需要的朋友们下面随着小... 目录一、Online DDL 是什么？二、Online DDL 的三种主要算法2.1COPY（复制法）
阅读更多...
mysql8.0.43使用InnoDB Cluster配置主从复制

mysql8.0.43使用InnoDB Cluster配置主从复制

《mysql8.0.43使用InnoDBCluster配置主从复制》本文主要介绍了mysql8.0.43使用InnoDBCluster配置主从复制,文中通过示例代码介绍的非常详细,对大家的学习或者... 目录1、配置Hosts解析(所有服务器都要执行)2、安装mysql shell(所有服务器都要执行)3、
阅读更多...
k8s中实现mysql主备过程详解

k8s中实现mysql主备过程详解

《k8s中实现mysql主备过程详解》文章讲解了在K8s中使用StatefulSet部署MySQL主备架构,包含NFS安装、storageClass配置、MySQL部署及同步检查步骤,确保主备数据一致... 目录一、k8s中实现mysql主备1.1 环境信息1.2 部署nfs-provisioner1.2.
阅读更多...
MySQL中VARCHAR和TEXT的区别小结

MySQL中VARCHAR和TEXT的区别小结

《MySQL中VARCHAR和TEXT的区别小结》MySQL中VARCHAR和TEXT用于存储字符串,VARCHAR可变长度存储在行内,适合短文本；TEXT存储在溢出页,适合大文本,下面就来具体的了解... 目录一、VARCHAR 和 TEXT 基本介绍1. VARCHAR2. TEXT二、VARCHAR
阅读更多...
MySQL中C接口的实现

MySQL中C接口的实现

《MySQL中C接口的实现》本节内容介绍使用C/C++访问数据库,包括对数据库的增删查改操作,主要是学习一些接口的调用,具有一定的参考价值,感兴趣的可以了解一下... 目录准备mysql库使用mysql库编译文件官方API文档对象的创建和关闭链接数据库下达sql指令select语句前言：本节内容介绍使用C/
阅读更多...
mybatis直接执行完整sql及踩坑解决

mybatis直接执行完整sql及踩坑解决

《mybatis直接执行完整sql及踩坑解决》MyBatis可通过select标签执行动态SQL,DQL用ListLinkedHashMap接收结果,DML用int处理,注意防御SQL注入,优先使用#... 目录myBATiFBNZQs直接执行完整sql及踩坑select语句采用count、insert、u
阅读更多...
MySQL之搜索引擎使用解读

MySQL之搜索引擎使用解读

《MySQL之搜索引擎使用解读》MySQL存储引擎是数据存储和管理的核心组件,不同引擎（如InnoDB、MyISAM）采用不同机制,InnoDB支持事务与行锁,适合高并发场景；MyISAM不支持事务,... 目录mysql的存储引擎是什么MySQL存储引擎的功能MySQL的存储引擎的分类查看存储引擎1.命令
阅读更多...
MyBatis Plus大数据量查询慢原因分析及解决

MyBatis Plus大数据量查询慢原因分析及解决

《MyBatisPlus大数据量查询慢原因分析及解决》大数据量查询慢常因全表扫描、分页不当、索引缺失、内存占用高及ORM开销,优化措施包括分页查询、流式读取、SQL优化、批处理、多数据源、结果集二次... 目录大数据量查询慢的常见原因优化方案高级方案配置调优监控与诊断总结大数据量查询慢的常见原因MyBAT
阅读更多...
一文详解MySQL索引(六张图彻底搞懂)

一文详解MySQL索引(六张图彻底搞懂)

《一文详解MySQL索引(六张图彻底搞懂)》MySQL索引的建立对于MySQL的高效运行是很重要的,索引可以大大提高MySQL的检索速度,：本文主要介绍MySQL索引的相关资料,文中通过代码介绍的... 目录一、什么是索引？为什么需要索引？二、索引该用哪种数据结构？1. 哈希表2. 跳表3. 二叉排序树4.
阅读更多...

Copyright China编程 23002807@qq.com

沪ICP备2023033072号-2