C# LINQ和Lambda表達(dá)式對(duì)照
1. 基本查詢語句
Linq語法：
var data=from a in db.Areas select a ;
Lamda語法：
var data=db.Areas;
sql語法：
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：
SELECT * FROM orderInfo WHERE orderId > 20
3. 關(guān)于“COUNT、SUM、MIN、MAX”函數(shù)的語句
Linq語法：
//查詢?cè)摫碇凶畲缶幪?hào)Id
var data=( from a in db.orderInfo select a ).Max( p=>p.orderId ) ;
//查詢?cè)摫碇凶钚【幪?hào)Id
var data=( from a in db.orderInfo select a ).Min( p=>p.orderId ) ;
//查詢?cè)摫頂?shù)據(jù)總條數(shù)
var data=( from a in db.orderInfo select a ).Count() ;
//查詢?cè)摫碇兴邢M(fèi)額的總數(shù)（求和）
var data=( from a in db.orderInfo select a ).Sum( p=>p.orderMoney ) ;
Lamda語法：
//查詢?cè)摫碇凶畲缶幪?hào)Id
var data=db.orderInfo.Max( t=>t.orderId );
//查詢?cè)摫碇凶钚【幪?hào)Id
var data=db.orderInfo.Min( t=>t.orderId );
//查詢?cè)摫頂?shù)據(jù)總條數(shù)
var data=db.orderInfo.Count();
//查詢?cè)摫碇兴邢M(fèi)額的總數(shù)（求和）
var data=db.orderInfo.Sum( t=>t.orderMoney);
sql語法：
SELECT MAX(orderId) FROM orderInfo
SELECT MIN(orderId) FROM orderInfo
SELECT COUNT(*) FROM orderInfo
SELECT SUM(orderMoney ) FROM orderInfo
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4. 關(guān)于數(shù)據(jù)排序的語句
Linq語法：
//倒序排序，升序可用ascending關(guān)鍵字
var data=from a in db.orderInfo
where a.orderId > 20
orderby a.orderId descending
select a ;
Lamda語法：
//情況一，根據(jù)單字段排序：
//倒序排序，升序可用OrderBy關(guān)鍵字
var data=db.orderInfo.OrderByDescending( t=>t.orderId ).
Where( t=>t.orderId > 20 ).
ToList();
//情況二，根據(jù)多字段主次排序：
//先按年升序，再按月升序
var priceMonthEntities = priceMonthApp.GetList().
OrderBy(t => t.F_Year).
ThenBy(t => t.F_Month).ToList();
sql語法：
//倒序排序，升序可用ASC關(guān)鍵字
SELECT * FROM orderInfo WHERE orderId > 20 ORDER BY orderId DESC
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5. 關(guān)于分頁查詢的語句
Linq語法：
var data=( from a in db.orderInfo select a ).
Skip((pageIndex-1) * pageSize).
Take(pageSize).
ToList();
Lamda語法：
//pageIndex:當(dāng)前頁碼，pageSize：分頁數(shù)據(jù)顯示條數(shù)
var data=db.orderInfo.Skip((pageIndex-1)* pageSize).Take(pageSize).ToList();
sql語法：
SELECT TOP pageSize * FROM orderInfo
WHERE orderId NOT IN(SELECT TOP( ( pageIndex - 1) * pageSize) orderId FROM orderInfo)
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6. 關(guān)于模糊查詢（like）的語句
Linq語法：
//使用Contains關(guān)鍵字進(jìn)行模糊匹配
var data= from a in db.orderInfo
where a.orderId.Contains(1)
select a;
Lamda語法：
//使用Contains關(guān)鍵字進(jìn)行模糊匹配
var data=db.orderInfo.Where(t=>t.F_UserId.Contains("1")).ToList();
sql語法：
//使用like關(guān)鍵字進(jìn)行模糊匹配
SELECT * FROM orderInfo WHERE orderId LIKE '%12%'
7. 關(guān)于分組查詢的語句
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,//分組字段
s.sMoney=s.Sum(a=>a.orderMoney),//分組后算出總的消費(fèi)額
s.maMoney=s.Max(a=>a.orderMoney),//分組后算出最大的消費(fèi)額
s.miMoney=s.Min(a=>a.orderMoney)//分組后算出最小的消費(fèi)額
};
Lamda語法：
//使用GroupBy關(guān)鍵字進(jìn)行分組查詢（單個(gè)字段）
var data=db.orderInfo.GroupBy(p => p.recType).
Select(t=>t.Key).ToList();
//使用GroupBy關(guān)鍵字進(jìn)行分組查詢（多個(gè)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語法：
SELECT orderType , SUM(orderMoney), MAX(orderMoney), MIN(orderMoney)
FROM orderInfo GROUP BY orderType
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8. 關(guān)于多表關(guān)聯(lián)查詢的語句
Linq語法：
//使用join關(guān)鍵字進(jìn)行表連接
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(guān)鍵字進(jìn)行表連接
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(guān)聯(lián)有多種方法，在此只舉一例）
SELECT * FROM orderInfo o ,orderType t
WHERE o.orderTypeId=t.id ORDER BY t.createDate
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9. 關(guān)于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：
SELECT * FROM orderInfo WHERE orderId IN (2213,43311,32422)
10. 關(guān)于去重查詢的語句
Linq語法：
//使用group關(guān)鍵字進(jìn)行表數(shù)據(jù)去重
var data= from a in db.orderInfo
group p by new {a.orderTypeId} into _group
select _group.FirstOrDefault();
//使用group關(guān)鍵字對(duì)多個(gè)字段進(jìn)行表數(shù)據(jù)去重
var data= from a in db.orderInfo
group p by new {a.orderTypeId,...} into _group
select _group.FirstOrDefault();
Lamda語法：
//單個(gè)去重：
// 使用GroupBy關(guān)鍵字進(jìn)行表數(shù)據(jù)去重
var data=db.orderInfo.GroupBy(t=>t.orderTypeId).Select(r => r.First()).ToList();
// 使用DistinctBy關(guān)鍵字進(jìn)行表數(shù)據(jù)去重
var data=db.orderInfo.DistinctBy(t=>t.orderTypeId).ToList();
//多個(gè)字段去重：
// 使用GroupBy關(guān)鍵字對(duì)多個(gè)字段進(jìn)行表數(shù)據(jù)去重
var data=db.orderInfo.GroupBy(t=>new{t.orderTypeId,...}).Select(r => r.First()).ToList();
// 使用DistinctBy關(guān)鍵字對(duì)多個(gè)字段進(jìn)行表數(shù)據(jù)去重
var data=db.orderInfo.DistinctBy(t=>new{t.orderTypeId,...}).ToList();
sql語法：
-- 使用DISTINCT關(guān)鍵字進(jìn)行表數(shù)據(jù)去重
SELECT DISTINCT orderTypeId FROM orderInfo
-- 使用GROUP BY關(guān)鍵字進(jìn)行表數(shù)據(jù)去重
SELECT orderTypeId FROM orderInfo GROUP BY orderTypeId
11. 內(nèi)連接 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：
SELECT r.* FROM Am_recProScheme AS r INNER JOIN Am_Test_Result AS t ON r.[rpId] = t.[rsId] ORDER BY r.[rpId] DESC
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12. 左連接 LEFT JOIN
要求：兩個(gè)DataTable關(guān)聯(lián)，查找只在第一個(gè)表中的數(shù)據(jù)
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(qiáng)");
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")
};
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}");
}
?
13. 三表連接
sql 語法：文章來源：http://www.zghlxwxcb.cn/news/detail-603986.html
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
Linq語法：文章來源地址http://www.zghlxwxcb.cn/news/detail-603986.html
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*/
//不要用對(duì)象的方式 因?yàn)閷?duì)象可能為null那么對(duì)象.屬性就會(huì)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); //字段為null不報(bào)異常
//Console.WriteLine(list[i].u1.name+'\t'+list[i].x1.jname+'\t'+list[i].v1.cname+"\r\n"); //對(duì)象x1 v1 有可能為null 拋異常
}
Console.ReadLine();
14. 實(shí)例用法：
//數(shù)據(jù)庫 + 自定義名稱 =new 數(shù)據(jù)庫
mydbDataContext con = new mydbDataContext();
//模糊查詢表達(dá)式中用.Contains
con.car.Where(r=>r.name.Contains(TextBox1.Text.Trim())).ToList(）;
//開頭查用.StartWith
con.car.Where(r => r.name.StartsWith(TextBox1.Text)).ToList();
//結(jié)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();
//上一頁，下一頁，組合查詢：
int PageCount = 5;//每頁顯示條數(shù)
//上一頁，PageCount_Label.Text為當(dāng)前頁碼
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();
//下一頁
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();
//組合查詢的點(diǎn)擊事件
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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