LINQ 查询（3）

内连接 Join

分别查询，查询数据再做连接

var racers = from r in Formula1.GetChampions()
                        from y in r.Years
                        select new
                        {
    
                            Year = y,
                            Name = r.FirstName + " " + r.LastName
                        };


            var teams = from t in Formula1.GetConstructorChampions()
                        from y in t.Years
                        select new
                        {
    
                            Year = y,
                            Name = t.Name
                        };

  

            var racersAndTeams = (from r in racers
                                 join t in teams on r.Year equals t.Year
                                  select new
                                  {
    
                                      r.Year,
                                      Champion = r.Name,
                                      Constructor = t.Name
                                  }).Take(10);

合并在一起，直击内连接

var racersAndTeams = (from r in
                      from r1 in Formula1.GetChampions()
                                      from yr in r1.Years
                                      select new
                                      {
    
                                          Year = yr,
                                          Name = r1.FirstName + " " + r1.LastName
                                     }
                                  join t in
                                  from t1 in Formula1.GetConstructorChampions()
                                  from yt in t1.Years
                                  select new
                                  {
    
                                      Year = yt,
                                      Name = t1.Name
                                  }
                                  on r.Year equals t.Year
                                  orderby t.Year
                                  select new
                                  {
    
                                      Year = r.Year,
                                      Racer = r.Name,
                                      Team = t.Name
                                  }).Take(10);

  

使用方法连接

var racers = Formula1.GetChampions()
                .SelectMany(r => r.Years, (r1, year) =>
                new
                {
    
                    Year = year,
                    Name = $"{
      r1.FirstName} {
      r1.LastName}"
                });

           var teams = Formula1.GetConstructorChampions()
                .SelectMany(t => t.Years, (t, year) =>
                new
                {
    
                    Year = year,
                    Name = t.Name
                });

            var racersAndTeams = racers.Join(teams,
                r => r.Year,
                t => t.Year,
                (r, t) =>
                new
                {
    
                    Year = r.Year,
                    Champion = r.Name,
                    Constructor = t.Name
                }).OrderBy(item => item.Year).Take(10);