如何让EF Framework通过查询生成此组

Question

我尝试了几种不同的方法，我对生成的输出查询印象不深。这似乎效率低下。它需要高效，不应该将所有行带回应用层

select year(datetaken) as yr,
       month(datetaken) as mth,
       day(datetaken) as dy,
       count(*) as totalpics
from photos
where photos.dateTaken <= @cutoffdate
group by year(datetaken), month(datetaken), day(datetaken)
order by yr asc, mth asc, dy asc

LINQ查询：

var query = ctx.Photos.Where(p => p.DateTaken <= maxCutOffDate)
    .GroupBy(p => new { p.DateTaken.Year, p.DateTaken.Month, p.DateTaken.Day })
    .Select(grp => grp);
var results = query.ToList();

Answer 1

仅选择分组键（grp.Key）：

var query = ctx.Photos.Where(p => p.DateTaken <= maxCutOffDate)
    .GroupBy(p => new { p.DateTaken.Year, p.DateTaken.Month, p.DateTaken.Day })
    .Select(grp => grp.Key);
var results = query.ToList();

修改

或者，包括totalpics的count：

var query = ctx.Photos.Where(p => p.DateTaken <= maxCutOffDate)
    .GroupBy(p => new { p.DateTaken.Year, p.DateTaken.Month, p.DateTaken.Day })
    .Select(grp => new
    {
        Date = grp.Key,
        TotalPics = grp.Count()
    });
var results = query.ToList();

Answer 2

您可以通过grp.Key访问关键属性，并通过grp.Count()访问。

var query = ctx.Photos.Where(p => p.DateTaken <= maxCutOffDate)
    .GroupBy(p => new { p.DateTaken.Year, p.DateTaken.Month, p.DateTaken.Day })
    .Select(grp => new 
                   { 
                       grp.Key.Year,
                       grp.Key.Month,
                       grp.Key.Day,
                       Count = grp.Count()
                   });
var results = query.ToList();