Select Linq 中 column1 和 column2 的所有不同的最后记录（不相应地工作）

Question

So I have a table like this:

Now I want distinct ShortCode order by the ID descending. In other words, the distinct last records. Like this:

So I tried GroupBy like:

var data = db.ShortCodes.GroupBy(x => x.ShortCode).Select(x => x.FirstOrDefault()).OrderByDescending(s=> s.ID);

This gave me distinct records but not the last ones, nor ordered by ID descending:

Now I also tried like suggested here

var data = db.ShortCodeManager
               .GroupBy(s => s. ShortCode)
               .Select(g => g.First())
               .OrderByDescending(s => s.ID);

This gave me the error The method 'First' can only be used as a final query operation. Consider using the method 'FirstOrDefault' in this instance instead. The method 'First' can only be used as a final query operation. Consider using the method 'FirstOrDefault' in this instance instead.

So I modified to FirstOrDefault() like:

var data = db.ShortCodeManager
               .GroupBy(s => s. ShortCode)
               .Select(g => g.FirstOrDefault())
               .OrderByDescending(s => s.ID);

This also gave me distinct records but not the last records:

So finally I tried like suggested here :

var data = db.ShortCodeManager.Where(a => a.ID > 0).GroupBy(x => x.ShortCode).OrderByDescending(grp => grp.Max(g => g.ID)).Select(a => a.FirstOrDefault());

Again, this gave me distinct records but not the last ones, nor ordered by ID descending:

So how am I to write the query to get the result I want in Linq? Also note, I need more of the distinct last records than ordering by ID descending. If anyone also knows how to write it in raw SQL it might be useful as well.

Answer 1

This LINQ query should work for your case:

var result = db.ShortCodeManager
    .GroupBy(x => x.ShortCode)
    .Select(gr => new { Id = gr.Max(g => g.Id), ShortCode = gr.Key})
    .ToList();

EDIT:

Based on your comment it looks like you need to cast anonymous object result to ShortCodeManagerModel type and then pass it to your view. So, somethin like this:

var result = db.ShortCodeManager
    .GroupBy(x => x.ShortCode)
    .Select(gr => new { Id = gr.Max(g => g.Id), ShortCode = gr.Key})
    .ToList();

var model = result 
    .Select(x => new ShortCodeManagerModel { Id = x.Id, ShortCode = x.ShortCode })
    .ToList();

And then pass model to you view.