C# LINQ语法详解

1.简单的linq语法

//
var ss = from r in db.Am_recProScheme
         select r;
//
var ss1 = db.Am_recProScheme;
//
string sssql = "select * from Am_recProScheme";

2.带where的查询

//
var ss = from r in db.Am_recProScheme
         where r.rpId >
         select r;
//
var ss1 = db.Am_recProScheme.Where(p => p.rpId > );
//
string sssql = "select * from Am_recProScheme where rpid>10";

3.简单的函数计算（count，min，max，sum）

//
////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);

//
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";

4.排序order by desc/asc

var ss = from r in db.Am_recProScheme
         where r.rpId >
         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 > ).ToList();
//倒序
var ss2 = db.Am_recProScheme.OrderByDescending(p => p.rpId).Where(p => p.rpId > ).ToList();
string sssql = "select * from Am_recProScheme where rpid>10 order by rpId [desc|asc]";

5.top(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();
//（）linq to ef 好像不支持 Last()
var ss1 = db.Am_recProScheme.FirstOrDefault();
//var ss1 = db.Am_recProScheme.First();
string sssql = "select top(1) * from Am_recProScheme";

6.跳过前面多少条数据取余下的数据

//
var ss = (from r in db.Am_recProScheme
          orderby r.rpId descending
          select r).Skip(); //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().ToList();
//
string sssql = "select * from  (select ROW_NUMBER()over(order by rpId desc) as rowNum, * from [Am_recProScheme]) as t where rowNum>10";

7.分页数据查询

//
var ss = (from r in db.Am_recProScheme
          where r.rpId >
          orderby r.rpId descending
          select r).Skip().Take(); //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 > ).Skip().Take().ToList();
//
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";

8.包含，类似like '%%'

//
var ss = from r in db.Am_recProScheme
         where r.SortsText.Contains("张")
         select r;
//
var ss1 = db.Am_recProScheme.Where(p => p.SortsText.Contains("张")).ToList();
//
string sssql = "select * from Am_recProScheme where SortsText like '%张%'";

9.分组group by

//
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);
}
//
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));
}
//
var ss2 = db.Am_recProScheme.GroupBy(p => p.recType);
foreach (var t in ss2)
{
    Response.Write(t.Key + "--" + t.Min(p => p.rpId));
}
//
string sssql = "select recType,min(rpId),max(rpId),sum(rpId) from Am_recProScheme group by recType";

10.sql中的In

//Linq
var ss = from p in db.Am_recProScheme
                  where (new int?[] { , , }).Contains(p.rpId)
                  select p;
foreach (var p in ss)
{
    Response.Write(p.Sorts);
}
//SQL
string st = "select * from Am_recProScheme where rpId in(24,25,26)";

11.内连接 INNER JOIN

//Linq
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;
//Lambda
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
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";

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(, "小明");
    dt1.Rows.Add(, "小红");
    dt1.Rows.Add(, "小黑");
    dt2.Columns.Add("ID", typeof(int));
    dt2.Columns.Add("Name", typeof(string));
    dt2.Rows.Add(, "小黄");
    dt2.Rows.Add(, "小红");
    dt2.Rows.Add(, "小强");
    //方法一：Linq
    //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：Lambda
    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();
}

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 = ; 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();

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 = ;//每页显示条数
//上一页，PageCount_Label.Text为当前页码
int pageNum = Convert.ToInt32(PageCount_Label.Text) - ;
Repeater1.DataSource = con.car.Skip((pageNum - ) * PageCount).Take(PageCount);
Repeater1.DataBind();
PageCount_Label.Text = pageNum.ToString();
//下一页
int pageNum = Convert.ToInt32(PageCount_Label.Text) + ;
Repeater1.DataSource = con.car.Skip((pageNum - ) * 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();