zoukankan      html  css  js  c++  java
  • Linq to sql(六):探究特性(一)

    延迟执行

           IQueryable query = from c in ctx.Customers select c;

           这样的查询句法不会导致语句立即执行,它仅仅是一个描述,对应一个SQL

    仅仅在需要使用的时候才会执行语句,比如:

            IQueryable query = from c in ctx.Customers select c;

            foreach (Customer c in query)

                Response.Write(c.CustomerID);

           如果你执行两次foreach操作,将会捕获到两次SQL语句的执行:

            IQueryable query = from c in ctx.Customers select c;

            foreach (Customer c in query)

                Response.Write(c.CustomerID);

            foreach (Customer c in query)

                Response.Write(c.ContactName);

           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]

     

    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]

           对于这样的需求,建议你先使用ToList()等方法把查询结果先进行保存,然后再对集合进行查询:

          IEnumerable<Customer> customers = (from c in ctx.Customers select c).ToList();

            foreach (Customer c in customers)

                Response.Write(c.CustomerID);

            foreach (Customer c in customers)

                Response.Write(c.ContactName);

           延迟执行的优点在于我们可以像拼接SQL那样拼接查询句法,然后再执行:

            var query = from c in ctx.Customers select c;

            var newquery = (from c in query select c).OrderBy(c => c.CustomerID);

  • 相关阅读:
    利用STM32播放音乐
    在MDK中使用$Sub$$和$Super$$的记录
    printf函数输出格式控制记录
    I2C软件实现
    C语言单项链表
    CreateEvent函数使用记录
    C语言宏定义使用记录
    GIT推送本地数据到远程空仓库
    2020-ECCV-Local Correlation Consistency for Knowledge Distillation阅读笔记
    2020-ECCV-Feature Normalized Knowledge Distillation for Image Classfication阅读笔记
  • 原文地址:https://www.cnblogs.com/kevin2013/p/1749059.html
Copyright © 2011-2022 走看看