初探Linq表达式和lambda表达式
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【Linq表达式】
LINQ,语言集成查询(LanguageIntegrated Query)是一组用于c#和Visual BasiC语言的扩展。它允许编写C#或者VisualBasic代码以查询数据库相同的方式操作内存数据。
博主认为,Linq表达式在用法方面就是SQL语句的变形,只要将SQL语句按照执行顺序来重新编写就可以得到Linq表达式。
【Lambda表达式】
Lambda 用在基于方法的 LINQ查询中,对网上查询到的关于Lambda表达式的资料不甚了解。Lambda表达式的使用在C#中,Lambda 表达式都使用 Lambda 运算符 =>,该运算符读为“goes to”。
博主认为:Lambda用于Linq查询中,Lambda表达式就是用Lambda语法将Linq表达式表示出来。
下面用几个语句对Linq表达式和Lambda表达式进行区别,为了便于理解,每个例子都以SQL语句、Linq表达式、Lambda表达式三种形式呈现。
1.简单的查询
- //1——Linq表达式
- var ss = from r in db.Am_recProScheme
- select r;
- //2——Lambda表达式
- var ss1 = db.Am_recProScheme;
- //3——SQL语句
- string sssql = "select * from Am_recProScheme";
- //1
- var ss = from r in db.Am_recProScheme
- where r.rpId > 10
- select r;
- //2
- var ss1 = db.Am_recProScheme.Where(p => p.rpId > 10);
- //3
- string sssql = "select * from Am_recProScheme where rpid>10";
- //1
- ////获取最大的rpId
- //var ss = (from r in db.Am_recProScheme
- // select r).Max(p => p.rpId);
- ////获取最小的rpId
- //var ss = (from r in db.Am_recProScheme
- // select r).Min(p => p.rpId);
- //获取结果集的总数
- //var ss = (from r in db.Am_recProScheme
- // select r).Count();
- //获取rpId的和
- var ss = (from r in db.Am_recProScheme
- select r).Sum(p => p.rpId);
- //2
- //var ss1 = db.Am_recProScheme.Max(p=>p.rpId);
- //var ss1 = db.Am_recProScheme.Min(p => p.rpId);
- //var ss1 = db.Am_recProScheme.Count() ;
- var ss1 = db.Am_recProScheme.Sum(p => p.rpId);
- Response.Write(ss);
- //3
- string sssql = "select max(rpId) from Am_recProScheme";
- sssql = "select min(rpId) from Am_recProScheme";
- sssql = "select count(1) from Am_recProScheme";
- sssql = "select sum(rpId) from Am_recProScheme";
- var ss = from r in db.Am_recProScheme
- where r.rpId > 10
- orderby r.rpId descending //倒序
- // orderby r.rpId ascending //正序
- select r;
- //正序
- var ss1 = db.Am_recProScheme.OrderBy(p => p.rpId).Where(p => p.rpId > 10).ToList();
- //倒序
- var ss2 = db.Am_recProScheme.OrderByDescending(p => p.rpId).Where(p => p.rpId > 10).ToList();
- <span style="font-family: Calibri; font-size: 18.6667px;">string sssql = "select * from Am_recProScheme where rpid>10 order by rpId [desc|asc]";</span>
- //1 如果取最后一个可以按倒叙排列再取值
- var ss = (from r in db.Am_recProScheme
- select r).FirstOrDefault();
- //2 linq to ef 好像不支持 Last()
- var ss1 = db.Am_recProScheme.FirstOrDefault();
- //var ss1 = db.Am_recProScheme.First();
- //3
- string sssql = "select top(1) * from Am_recProScheme";
- //1
- var ss = (from r in db.Am_recProScheme
- orderby r.rpId descending
- select r).Skip(10); //跳过前10条数据,取10条之后的所有数据
- //2
- var ss1 = db.Am_recProScheme.OrderByDescending(p => p.rpId).Skip(10).ToList();
- //3
- string sssql = "select * from (select ROW_NUMBER()over(order by rpId desc) as rowNum, * from [Am_recProScheme]) as t where rowNum>10";
- //1
- var ss = (from r in db.Am_recProScheme
- where r.rpId > 10
- orderby r.rpId descending
- select r).Skip(10).Take(10); //取第11条到第20条数据
- //2 Take(10): 数据从开始获取,获取指定数量(10)的连续数据
- var ss1 = db.Am_recProScheme.OrderByDescending(p => p.rpId).Where(p => p.rpId > 10).Skip(10).Take(10).ToList();
- //3
- string sssql = "select * from (select ROW_NUMBER()over(order by rpId desc) as rowNum, * from [Am_recProScheme]) as t where rowNum>10 and rowNum<=20";
- //1
- var ss = from r in db.Am_recProScheme
- where r.SortsText.Contains("张")
- select r;
- //2
- var ss1 = db.Am_recProScheme.Where(p => p.SortsText.Contains("张")).ToList();
- //3
- string sssql = "select * from Am_recProScheme where SortsText like '%张%'";
- //1
- var ss = from r in db.Am_recProScheme
- orderby r.rpId descending
- group r by r.recType into n
- select new
- {
- n.Key, //这个Key是recType
- rpId = n.Sum(r => r.rpId), //组内rpId之和
- MaxRpId = n.Max(r => r.rpId),//组内最大rpId
- MinRpId = n.Min(r => r.rpId), //组内最小rpId
- };
- foreach (var t in ss)
- {
- Response.Write(t.Key + "--" + t.rpId + "--" + t.MaxRpId + "--" + t.MinRpId);
- }
- //2
- var ss1 = from r in db.Am_recProScheme
- orderby r.rpId descending
- group r by r.recType into n
- select n;
- foreach (var t in ss1)
- {
- Response.Write(t.Key + "--" + t.Min(p => p.rpId));
- }
- //3
- var ss2 = db.Am_recProScheme.GroupBy(p => p.recType);
- foreach (var t in ss2)
- {
- Response.Write(t.Key + "--" + t.Min(p => p.rpId));
- }
- //4
- string sssql = "select recType,min(rpId),max(rpId),sum(rpId) from Am_recProScheme group by recType";
- //1
- var ss = from r in db.Am_recProScheme
- join w in db.Am_Test_Result on r.rpId equals w.rsId
- orderby r.rpId descending
- select r;
- //2
- var ss1 = db.Am_recProScheme.Join(db.Am_Test_Result, p => p.rpId, r => r.rsId, (p, r) => p).OrderByDescending(p => p.rpId).ToList();
- //3
- string sssql = "select r.* from [Am_recProScheme] as r inner join [dbo].[Am_Test_Result] as t on r.[rpId] = t.[rsId] order by r.[rpId] desc";
- //1——Linq
- var ss = from p in db.Am_recProScheme
- where (new int?[] { 24, 25,26 }).Contains(p.rpId)
- select p;
- foreach (var p in ss)
- {
- Response.Write(p.Sorts);
- }
- //2——SQL
- string st = "select * from Am_recProScheme where rpId in(24,25,26)";
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