C# LINQ和Lambda表达式详解
2021/12/7 11:17:49
本文主要是介绍C# LINQ和Lambda表达式详解,对大家解决编程问题具有一定的参考价值,需要的程序猿们随着小编来一起学习吧!
引用网址: https://www.cnblogs.com/zhaoshujie/p/10434657.html
(1) 简单的查询语句
Linq语法: var data=from a in db.Areas select a ; Lamda语法: var data=db.Areas; sql语法: string sqlStr=" SELECT * FROM Areas ";
(2) 简单的WHERE语句
Linq语法: var data=from a in db.orderInfo where a.orderId > 20 select a ; Lamda语法: var data=db.orderInfo.Where( t=>t.orderId > 20 ) ; sql语法: string sqlStr=" SELECT * FROM orderInfo WHERE orderId > 20 ";
(3) 关于“COUNT、SUM、MIN、MAX”函数的语句
Linq语法: var data=( from a in db.orderInfo select a ).Max( p=>p.orderId ) ;//查询该表中最大编号Id var data=( from a in db.orderInfo select a ).Min( p=>p.orderId ) ;//查询该表中最小编号Id var data=( from a in db.orderInfo select a ).Count() ;//查询该表数据总条数 var data=( from a in db.orderInfo select a ).Sum( p=>p.orderMoney ) ;//查询该表中所有消费额的总数(求和) Lamda语法: var data=db.orderInfo.Max( t=>t.orderId );//查询该表中最大编号Id var data=db.orderInfo.Min( t=>t.orderId );//查询该表中最小编号Id var data=db.orderInfo.Count();//查询该表数据总条数 var data=db.orderInfo.Sum( t=>t.orderMoney );//查询该表中所有消费额的总数(求和) sql语法: string sqlStr=" SELECT MAX(orderId) FROM orderInfo "; string sqlStr=" SELECT MIN(orderId) FROM orderInfo "; string sqlStr=" SELECT COUNT(*) FROM orderInfo "; string sqlStr=" SELECT SUM(orderMoney ) FROM orderInfo ";
(4) 关于数据排序的语句
Linq语法: var data=from a in db.orderInfo where a.orderId > 20 orderby a.orderId descending select a ;//倒序排序,升序可用ascending关键字 Lamda语法: //情况一,根据单字段排序: var data=db.orderInfo.OrderByDescending( t=>t.orderId ).Where( t=>t.orderId > 20 ) .ToList();//倒序排序,升序可用OrderBy关键字 //情况二,根据多字段主次排序: var priceMonthEntities = priceMonthApp.GetList().OrderBy(t => t.F_Year).ThenBy(t => t.F_Month).ToList();//先按年升序,再按月升序 sql语法: string sqlStr=" SELECT * FROM orderInfo WHERE orderId > 20 ORDER BY orderId DESC ";//倒序排序,升序可用ASC关键字
(5) 关于分页查询的语句
Linq语法: var data=( from a in db.orderInfo select a ) .Skip((pageIndex-1) * pageSize).Take(pageSize).ToList(); Lamda语法: var data=db.orderInfo.Skip((pageIndex-1)* pageSize).Take(pageSize).ToList();;//pageIndex:当前页码,pageSize:分页数据显示条数 sql语法: string sqlStr="SELECT TOP pageSize * FROM orderInfo WHERE orderId NOT IN(SELECT TOP( ( pageIndex - 1) * pageSize) orderId FROM orderInfo)";
(6) 关于模糊查询(like)的语句
Linq语法: var data= from a in db.orderInfo where a.orderId.Contains(1) select a;//使用Contains关键字进行模糊匹配 Lamda语法: var data=db.orderInfo.Where(t=>t.F_UserId.Contains("1")).ToList();//使用Contains关键字进行模糊匹配 sql语法: string sqlStr="SELECT * FROM orderInfo WHERE orderId LIKE '%12%'";//使用like关键字进行模糊匹配
(7) 关于分组查询的语句
Linq语法: var data= from a in db.orderInfo orderby a.orderId descending group a by a.orderType into s select new{ s.key,//分组字段 s.sMoney=s.Sum(a=>a.orderMoney),//分组后算出总的消费额 s.maMoney=s.Max(a=>a.orderMoney),//分组后算出最大的消费额 s.miMoney=s.Min(a=>a.orderMoney)//分组后算出最小的消费额 }; Lamda语法: //使用GroupBy关键字进行分组查询(单个字段) var data=db.orderInfo.GroupBy(p => p.recType).Select(t=>t.Key).ToList(); //使用GroupBy关键字进行分组查询(多个字段) var data=db.orderInfo.GroupBy(p =>new{ p.recType,p.orderId}).Select(t=>new{ recType=t.Key.recType,orderId=t.Key.orderId}).ToList(); sql语法: string sqlStr="SELECT orderType , SUM(orderMoney), MAX(orderMoney), MIN(orderMoney) FROM orderInfo GROUP BY orderType";
(8) 关于多表关联查询的语句
Linq语法: //使用join关键字进行表连接 var data= from a in db.orderInfo join e in db.orderType on a.orderTypeId equals e.id select r ; var query=from t in db.orderInfo join s in db.orderType on t.orderTypeId equals s.id select new { orderId=t.id, orderTypeName=s.name, ... } Lamda语法: var data=db.orderInfo.Join(db.orderType,t=>t.orderTypeId,s=>s.id,(t,s)=>t).OrderByDescending(t=>t.orderId). Select( t=> new{ orderId=t.t.id, orderTypeName=t.s.name, ... }).ToList(); //使用Join关键字进行表连接 EF Core中的写法: var data=db.orderInfo.Join(db.orderType,t=>t.orderTypeId,s=>s.id,(t,s)=>new{ orderId=s.Id, ..... }).toList(); sql语法:(sql语句表关联有多种方法,在此只举一例) string sqlStr="SELECT * FROM orderInfo o ,orderType t WHERE o.orderTypeId=t.id ORDER BY t.createDate";
(9) 关于in查询的语句
Linq语法: var data= from a in db.orderInfo where (new int?[2213,43311,32422]).Contains(a.orderId) select a ; Lamda语法: var data=db.orderInfo.Where(t=>(new int?[2213,43311,32422]).Contains(t.orderId)).ToList(); sql语法: string sqlStr="SELECT * FROM orderInfo WHERE orderId IN (2213,43311,32422)";
(10) 关于去重查询的语句
Linq语法: var data= from a in db.orderInfo group p by new {a.orderTypeId} into _group select _group.FirstOrDefault(); //使用group关键字进行表数据去重 var data= from a in db.orderInfo group p by new {a.orderTypeId,...} into _group select _group.FirstOrDefault(); //使用group关键字对多个字段进行表数据去重 Lamda语法: //单个去重: var data=db.orderInfo.GroupBy(t=>t.orderTypeId).Select(r => r.First()).ToList(); //使用GroupBy关键字进行表数据去重 var data=db.orderInfo.DistinctBy(t=>t.orderTypeId).ToList(); //使用DistinctBy关键字进行表数据去重 //多个字段去重: var data=db.orderInfo.GroupBy(t=>new{t.orderTypeId,...}).Select(r => r.First()).ToList(); //使用GroupBy关键字对多个字段进行表数据去重 var data=db.orderInfo.DistinctBy(t=>new{t.orderTypeId,...}).ToList(); //使用DistinctBy关键字对多个字段进行表数据去重 sql语法: string sqlStr="SELECT DISTINCT orderTypeId FROM orderInfo";//使用DISTINCT关键字进行表数据去重 string sqlStr="SELECT orderTypeId FROM orderInfo GROUP BY orderTypeId";//使用GROUP BY关键字进行表数据去重
(11) 内连接 INNER JOIN
Linq语法: 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; Lamda语法: var ss1 = db.Am_recProScheme.Join(db.Am_Test_Result, p => p.rpId, r => r.rsId, (p, r) => p).OrderByDescending(p => p.rpId).ToList(); sql语法: string sssql = "SELECT r.* FROM Am_recProScheme AS r INNER JOIN Am_Test_Result AS t ON r.[rpId] = t.[rsId] ORDER BY r.[rpId] DESC";
(12) 左连接 LEFT JOIN
//两个DataTable关联,查找只在第一个表中的数据 static void Main(string[] args) { DataTable dt1 = new DataTable("Ta"); DataTable dt2 = new DataTable("Tb"); dt1.Columns.Add("ID", typeof(int)); dt1.Columns.Add("Name", typeof(string)); dt1.Rows.Add(1, "小明"); dt1.Rows.Add(2, "小红"); dt1.Rows.Add(3, "小黑"); dt2.Columns.Add("ID", typeof(int)); dt2.Columns.Add("Name", typeof(string)); dt2.Rows.Add(1, "小黄"); dt2.Rows.Add(2, "小红"); dt2.Rows.Add(3, "小强"); 方法一:Linq语法 var query = from q1 in dt1.AsEnumerable() join q2 in dt2.AsEnumerable() on q1.Field<string>("Name") equals q2.Field<string>("Name") into tmp from q3 in tmp.DefaultIfEmpty() where q3 == null select new { ID = q1.Field<int>("ID"), Name = q1.Field<string>("Name") }; 方法二:Lamda语法 var query = dt1.AsEnumerable().GroupJoin( dt2.AsEnumerable(), x => x.Field<string>("Name"), y => y.Field<string>("Name"), (x, y) => y.DefaultIfEmpty(). Where(w => w == null). Select(z => new { ID = x.Field<int>("ID"), Name = x.Field<string>("Name") }) ).SelectMany(x => x); foreach (var item in query) { Console.WriteLine($"ID={item.ID} Name={item.Name}"); } Console.Read(); }
(13) 三表连接
SELECT id, name, jname, cname FROM userinfo u LEFT JOIN job j on u.job = j.jid LEFT JOIN city c on u.city = c.cid var list = ( from u in dc.userinfos join j in dc.jobs on u.job equals j.jid into j_join from x in j_join.DefaultIfEmpty() join c in dc.cities on u.city equals c.cid into c_join from v in c_join.DefaultIfEmpty() select new { id = u.id, name = u.name, jname = x.jname, cname = v.cname, /*u1=u,x1=x,v1=v*/ //不要用对象的方式 因为对象可能为null那么对象.属性就会抛异常 } ).ToList(); for (var i = 0; i < list.Count(); i++) { Console.WriteLine(list[i].name + '\t' + list[i].jname + '\t' + list[i].cname); //字段为null不报异常 //Console.WriteLine(list[i].u1.name+'\t'+list[i].x1.jname+'\t'+list[i].v1.cname+"\r\n"); //对象x1 v1 有可能为null 抛异常 } Console.ReadLine();
(14) 实例用法:
//数据库 + 自定义名称 =new 数据库 mydbDataContext con = new mydbDataContext(); //模糊查询表达式中用.Contains con.car.Where(r=>r.name.Contains(TextBox1.Text.Trim())).ToList(); //开头查用.StartWith con.car.Where(r => r.name.StartsWith(TextBox1.Text)).ToList(); //结尾查用.EndWith con.car.Where(r => r.name.EndsWith(TextBox1.Text)).ToList(); //最大值 con.car.Max(r => r.price * r.oil).ToString(); //最小值 con.car.Min(r => r.price).ToString(); //求和 con.car.Sum(r => r.price).ToString(); //平均值 con.car.Average(r => r.price).ToString(); //升序: con.car.OrderBy(r => r.price).ToList(); //降序: con.car.OrderByDescending(r => r.price).ToList(); //上一页,下一页,组合查询: int PageCount = 5;//每页显示条数 //上一页,PageCount_Label.Text为当前页码 int pageNum = Convert.ToInt32(PageCount_Label.Text) - 1; Repeater1.DataSource = con.car.Skip((pageNum - 1) * PageCount).Take(PageCount); Repeater1.DataBind(); PageCount_Label.Text = pageNum.ToString(); //下一页 int pageNum = Convert.ToInt32(PageCount_Label.Text) + 1; Repeater1.DataSource = con.car.Skip((pageNum - 1) * PageCount).Take(PageCount); Repeater1.DataBind(); PageCount_Label.Text = pageNum.ToString(); //组合查询的点击事件 List<car> list = con.car.ToList(); if (TextBox2.Text != "") { List<car> list1 = con.car.Where(r => r.name.Contains(TextBox2.Text)).ToList(); list = list.Intersect(list1).ToList(); } if (TextBox3.Text != "") { List<car> list1 = con.car.Where(r => r.oil == Convert.ToDecimal(TextBox3.Text)).ToList(); list = list.Intersect(list1).ToList(); } if (TextBox4.Text != "") { List<car> list1 = con.car.Where(r => r.powers == Convert.ToInt32(TextBox4.Text)).ToList(); list = list.Intersect(list1).ToList(); } Repeater1.DataSource = list; Repeater1.DataBind();
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