C# LINQ和Lambda表达式详解

(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();