高级投影查询

本文关键字：查询 投影 高级 | 更新日期: 2023-09-27 18:35:09

有什么更好的方法可以做到这一点吗

using (var db = new SmartContext())
{
    var MyQuery = from idr in db.ID_SB
                  join tk in db.Track_SB on idr.MB_Track_ID equals tk.MB_TrackID
                  join talr in db.Track_Album_Reln on tk.MB_TrackID equals talr.MB_Track_ID
                  join tal in db.Album_SB on talr.MB_AlbumID equals tal.MB_Release_ID
                  orderby idr.Last_played descending
                  select new
                  {
                      mb = tk.MB_TrackID,
                      Hash = idr.Hash,
                      Title = tk.Title,
                      Album = tal.Album_Name,
                      Times_Played = idr.Times_played,
                      Last_Played = idr.Last_played
                  };
    string artist = "";
    var list = new[] { new {  Hash = "", Title = "", Album = "", Artist = "", Times_Played = "", Last_Played = ""}}.ToList();
    list.Clear();
    foreach (var q in MyQuery)
    {
        int i = 0;
        var art = db.Track_Artist_Reln.Where(a => a.MB_Track_ID == q.mb);
        foreach (var a in art)
        {
            var tart = db.Artist_SB.Where(ar => ar.MB_Artist_ID == a.MB_ArtistID).Select(ar => ar.Artist_Name);
            foreach (var tar in tart)
            { if (i != 0) { artist = artist + ", " + tar; } else { artist = tar; i++; } }
        }
        list.Add(new
        {
            Hash = q.Hash.ToString(),
            Title = q.Title.ToString(),
            Album = q.Album.ToString(),
            Artist = artist.ToString(),
            Times_Played = q.Times_Played.ToString(),
            Last_Played = q.Last_Played.ToString(),            
          });
    }

    ListView1.ItemsSource = list;
}

有没有更好的方法，因为它会消耗大量时间

此代码用于目的

在此我想检索

  • 散 列
  • 标题
  • 专辑名称
    • 艺术家
  • （在单行字段艺术家中包含该歌曲的所有艺术家）
  • 播放次数
  • 上次播放时间

除了艺术家名字之外的所有内容，我只能通过加入所有内容来获得一切

有很多五月关系主表ID_SB、Track_SB、Album_SB、Artist_SB其他是存储两个表的主键的关系

高级投影查询

内部循环可以替换为使用 string.Join(…) 和 .SelectMany() 将嵌套列表（实际上IEnumerable）展平为一个：

var artist = string.Join(", ",
                         db.Track_Artist_Reln
                           .Where(a => a.MB_Track_ID == q.mb)
                           .SelectMany(a => db.Artist_SB
                                              .Where(ar => ar.MB_Artist_ID == a.MB_ArtistID)
                                              .Select(ar => ar.Artist_Name)));

您可以使用.ToList()，而不是手动将枚举对象转换为列表：

var list = MyQuery.Select(q => new{
        Hash = q.Hash.ToString(),
        Title = q.Title.ToString(),
        Album = q.Album.ToString(),
        Artist = string.Join(", ",
                             db.Track_Artist_Reln
                               .Where(a => a.MB_Track_ID == q.mb)
                               .SelectMany(a => db.Artist_SB
                                                  .Where(ar => ar.MB_Artist_ID == a.MB_ArtistID)
                                                  .Select(ar => ar.Artist_Name))),
        Times_Played = q.Times_Played.ToString(),
        Last_Played = q.Last_Played.ToString(),            
        }.ToList();
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