C# LINQ和Lambda表示式詳解
阿新 • • 發佈:2021-07-13
(1) 簡單的查詢語句
Linq語法:
var data=from a in db.Areas select a ;
Lamda語法:
var data=db.Areas;
sql語法:
string sqlStr=" SELECT * FROM Areas ";
(2) 簡單的WHERE語句
Linq語法:
var data=from a in db.orderInfo where a.orderId > 20 select a ;
Lamda語法:
var data=db.orderInfo.Where( t=>t.orderId > 20 ) ;
sql語法:
string sqlStr=" SELECT * FROM orderInfo WHERE orderId > 20 ";
(3) 關於“COUNT、SUM、MIN、MAX”函式的語句
Linq語法:
var data=( from a in db.orderInfo select a ).Max( p=>p.orderId ) ;//查詢該表中最大編號Id
var data=( from a in db.orderInfo select a ).Min( p=>p.orderId ) ;//查詢該表中最小編號Id
var data=( from a in db.orderInfo select a ).Count() ;//查詢該表資料總條數
var data=( from a in db.orderInfo select a ).Sum( p=>p.orderMoney ) ;//查詢該表中所有消費額的總數(求和)
Lamda語法:
var data=db.orderInfo.Max( t=>t.orderId );//查詢該表中最大編號Id
var data=db.orderInfo.Min( t=>t.orderId );//查詢該表中最小編號Id
var data=db.orderInfo.Count();//查詢該表資料總條數
var data=db.orderInfo.Sum( t=>t.orderMoney );//查詢該表中所有消費額的總數(求和)
sql語法:
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) 關於資料排序的語句
Linq語法:
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關鍵字
Lamda語法:
//情況一,根據單欄位排序:
var data=db.orderInfo.OrderByDescending( t=>t.orderId ).Where( t=>t.orderId > 20 ) .ToList();//倒序排序,升序可用OrderBy關鍵字
//情況二,根據多欄位主次排序:
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語法:
string sqlStr=" SELECT * FROM orderInfo WHERE orderId > 20 ORDER BY orderId DESC ";//倒序排序,升序可用ASC關鍵字
(5) 關於分頁查詢的語句
Linq語法:
var data=( from a in db.orderInfo select a ) .Skip((pageIndex-1) * pageSize).Take(pageSize).ToList();
Lamda語法:
var data=db.orderInfo.Skip((pageIndex-1)* pageSize).Take(pageSize).ToList();;//pageIndex:當前頁碼,pageSize:分頁資料顯示條數
sql語法:
string sqlStr="SELECT TOP pageSize * FROM orderInfo WHERE orderId NOT IN(SELECT TOP( ( pageIndex - 1) * pageSize) orderId FROM orderInfo)";
(6) 關於模糊查詢(like)的語句
Linq語法:
var data= from a in db.orderInfo where a.orderId.Contains(1) select a;//使用Contains關鍵字進行模糊匹配
Lamda語法:
var data=db.orderInfo.Where(t=>t.F_UserId.Contains("1")).ToList();//使用Contains關鍵字進行模糊匹配
sql語法:
string sqlStr="SELECT * FROM orderInfo WHERE orderId LIKE '%12%'";//使用like關鍵字進行模糊匹配
(7) 關於分組查詢的語句
Linq語法:
var data= from a in db.orderInfo orderby a.orderId descending
group a by a.orderType into s select new{
s.key,//分組欄位
s.sMoney=s.Sum(a=>a.orderMoney),//分組後算出總的消費額
s.maMoney=s.Max(a=>a.orderMoney),//分組後算出最大的消費額
s.miMoney=s.Min(a=>a.orderMoney)//分組後算出最小的消費額
};
Lamda語法:
//使用GroupBy關鍵字進行分組查詢(單個欄位)
var data=db.orderInfo.GroupBy(p => p.recType).Select(t=>t.Key).ToList();
//使用GroupBy關鍵字進行分組查詢(多個欄位)
var data=db.orderInfo.GroupBy(p =>new{ p.recType,p.orderId}).Select(t=>new{ recType=t.Key.recType,orderId=t.Key.orderId}).ToList();
sql語法:
string sqlStr="SELECT orderType , SUM(orderMoney), MAX(orderMoney), MIN(orderMoney) FROM orderInfo GROUP BY orderType";
(8) 關於多表關聯查詢的語句
Linq語法:
//使用join關鍵字進行表連線
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語法:
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關鍵字進行表連線
EF Core中的寫法:
var data=db.orderInfo.Join(db.orderType,t=>t.orderTypeId,s=>s.id,(t,s)=>new{
orderId=s.Id,
.....
}).toList();
sql語法:(sql語句表關聯有多種方法,在此只舉一例)
string sqlStr="SELECT * FROM orderInfo o ,orderType t WHERE o.orderTypeId=t.id ORDER BY t.createDate";
(9) 關於in查詢的語句
Linq語法:
var data= from a in db.orderInfo where (new int?[2213,43311,32422]).Contains(a.orderId) select a ;
Lamda語法:
var data=db.orderInfo.Where(t=>(new int?[2213,43311,32422]).Contains(t.orderId)).ToList();
sql語法:
string sqlStr="SELECT * FROM orderInfo WHERE orderId IN (2213,43311,32422)";
(10) 關於去重查詢的語句
Linq語法:
var data= from a in db.orderInfo group p by new {a.orderTypeId} into _group select _group.FirstOrDefault(); //使用group關鍵字進行表資料去重
var data= from a in db.orderInfo group p by new {a.orderTypeId,...} into _group select _group.FirstOrDefault(); //使用group關鍵字對多個欄位進行表資料去重
Lamda語法:
//單個去重:
var data=db.orderInfo.GroupBy(t=>t.orderTypeId).Select(r => r.First()).ToList(); //使用GroupBy關鍵字進行表資料去重
var data=db.orderInfo.DistinctBy(t=>t.orderTypeId).ToList(); //使用DistinctBy關鍵字進行表資料去重
//多個欄位去重:
var data=db.orderInfo.GroupBy(t=>new{t.orderTypeId,...}).Select(r => r.First()).ToList(); //使用GroupBy關鍵字對多個欄位進行表資料去重
var data=db.orderInfo.DistinctBy(t=>new{t.orderTypeId,...}).ToList(); //使用DistinctBy關鍵字對多個欄位進行表資料去重
sql語法:
string sqlStr="SELECT DISTINCT orderTypeId FROM orderInfo";//使用DISTINCT關鍵字進行表資料去重
string sqlStr="SELECT orderTypeId FROM orderInfo GROUP BY orderTypeId";//使用GROUP BY關鍵字進行表資料去重
(11) 內連線 INNER JOIN
Linq語法:
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語法:
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語法:
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) 左連線 LEFT JOIN
//兩個DataTable關聯,查詢只在第一個表中的資料
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, "小紅");
dt1.Rows.Add(3, "小黑");
dt2.Columns.Add("ID", typeof(int));
dt2.Columns.Add("Name", typeof(string));
dt2.Rows.Add(1, "小黃");
dt2.Rows.Add(2, "小紅");
dt2.Rows.Add(3, "小強");
方法一:Linq語法
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語法
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) 三表連線
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*/
//不要用物件的方式 因為物件可能為null那麼物件.屬性就會拋異常
}
).ToList();
for (var i = 0; i < list.Count(); i++)
{
Console.WriteLine(list[i].name + '\t' + list[i].jname + '\t' + list[i].cname); //欄位為null不報異常
//Console.WriteLine(list[i].u1.name+'\t'+list[i].x1.jname+'\t'+list[i].v1.cname+"\r\n"); //物件x1 v1 有可能為null 拋異常
}
Console.ReadLine();
轉自:https://www.cnblogs.com/zhaoshujie/p/10434657.html