LINQ和EntityFramework：在分组后得到第一个结果

Question

What i have is the next table:

What im trying to do in LINQ using C# is to group by the Created column (ignoring the time!) and showing its counts next to each date so i could grab any top rows i want. This query i got no problem doing in sql like the following way:

SELECT CAST(Created AS DATE) 'Created', COUNT(*) AS 'Count'
FROM Alert
GROUP BY CAST(Created AS DATE)
ORDER BY 'Count' DESC

which results with:

but again, i'd like to do that in LINQ and simply all my tryouts failed, could anyone please guide me?

Answer 1

Use the Date property of DateTime :

var query = db.Alert
    .GroupBy(a => a.Created.Date)
    .Select(g => new { Created  = g.Key, Count = g.Count() })
    .OrderByDescending(x => x.Count);

It seems that Linq-To-Entities does not support the DateTime.Date property. So you have to use DbFunctions.TruncateTime .

Then this should work:

var query = db.Alert
    .GroupBy(a => DbFunctions.TruncateTime(a.Created))
    .Select(g => new { Created  = g.Key, Count = g.Count() })
    .OrderByDescending(x => x.Count);

Answer 2

@TimSchmelter is right, but I would like to remind that in Linq you can do something like :

var multimap = db.Alert.ToLookup(o => o.Created.Date, o => o);

The count of items is just property of the value list you get when you use a date as a key.

foreach(date in multimap.Select(g => g.Key).ToList()){
    var itemCount = multimap[date].Count();