(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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