c# - 我如何最优雅地将聚合 SQL 的左连接表示为 LINQ 查询

Question

SQL：

SELECT
   u.id,
   u.name,
   isnull(MAX(h.dateCol), '1900-01-01') dateColWithDefault
FROM universe u
LEFT JOIN history h 
   ON u.id=h.id 
   AND h.dateCol<GETDATE()-1
GROUP BY u.Id, u.name

Answer 1

一种解决方案，尽管将处理 null 值推迟到代码，可能是：

昨天的日期时间 = DateTime.Now.Date.AddDays(-1);

var collection=
    from u in db.Universe
    select new
    {
        u.id,
        u.name,
        MaxDate =(DateTime?)
       (
           from h in db.History
           where u.Id == h.Id
           && h.dateCol < yesterday
           select h.dateCol 
       ).Max()
    };

这不会产生完全相同的 SQL，但会提供相同的逻辑结果。将“复杂”的 SQL 查询转换为 LINQ 并不总是那么简单。

Answer 2

var collection=
    from u in db.Universe
    select new
    {
        u.id,
        u.name,
        MaxDate =(DateTime?)
       (
           from h in db.History
           where u.Id == h.Id
           && h.dateCol < yesterday
           select h.dateCol 
       ).Max()
    };

Just youse the above code and this should work fine!

Answer 3

这不是您的完整答案，但在左侧连接块上，您可以使用 DefaultIfEmpty 运算符，如下所示：

var collection = 
from u in db.Universe
join history in db.History on u.id = history.id into temp
from h in temp.DefaultIfEmpty()
where h.dateCol < DateTime.Now.Date.AddDays(-1)
select u.id, u.name, h.dateCol ?? '1900-01-01'

I haven't had the need to do any groupby commands yet, so I left that out as to not send you down the wrong path. Two other quick things to note. I have been unable to actually join on two parameters although as above there are ways to get around it. Also, the ?? operator works really well in place of the isnull in SQL.

Answer 4

You're going to want to use the join into construct to create a group query.

TestContext db = new TestContext(CreateSparqlTripleStore());
var q = from a in db.Album
        join t in db.Track on a.Name equals t.AlbumName into tracks
        select new Album{Name = a.Name, Tracks = tracks};
foreach(var album in q){
    Console.WriteLine(album.Name);
    foreach (Track track in album.Tracks)
    {
        Console.WriteLine(track.Title);
    }
}