针对实体框架使用 Linq 显示等效于 SQL pivot 表

Question

I've been struggling for a few days to display something like Pivoting a dynamic table on SQL, using Linq. I don't actually want to create a pivoted table, I just want to display the results as if it was a pivoted table.

I have two entities:

Category:

 public int Id { get; set; }
        public string Name { get; set; }
        public string Icon { get; set; }
        public ICollection<StaticEvent> StaticEvents { get; set; }

StaticEvent:

public int Id { get; set; }
        public int CategoryId {get; set; }
        public Category Category { get; set; }
        public DateTimeOffset Time {get; set; }

        [MaxLength(500)]
        public string Comment {get; set;}

I'm trying to generate a table showing the COUNT of STATIC EVENTS per CATEGORY PER YEARMONTH. So the rows would be the YEARMONTH, the columns would be the Categories and the values would be the COUNT.

I got to the point where I can count the STATIC EVENTS per YEARMONTH:

var query = _context.Categories
            .SelectMany(c => c.StaticEvents )
            .GroupBy(c => 
               new {
                   Year = c.Time.Year,
                   Month = c.Time.Month
               })
            .Select( x=> new {YearMonth = x.Key, Count = x.Count()})
            .ToList();

            foreach (var x in query)
            {Console.WriteLine($"Month = {x.YearMonth.Year},{x.YearMonth.Month},  , Count = " + x.Count);}

but I'm lost about what to do from here.

Answer 1

If Categories is unknown values which are stored in database, it is not possible to do that via LINQ without dynamic query creation.

There is query which do the job when you known categories on the compile time. Since you have not specified EF version, I have emulated Count with Sum :

var query = _context.Categories
    .SelectMany(c => c.StaticEvents)
    .GroupBy(c => 
        new {
            Year = c.Time.Year,
            Month = c.Time.Month
        })
    .Select(x => new {
        YearMonth = x.Key, 
        Cat1 = x.Sum(z => z.CategoryId == 1 ? 1 : 0),
        Cat2 = x.Sum(z => z.CategoryId == 2 ? 1 : 0),
        Cat3 = x.Sum(z => z.CategoryId == 3 ? 1 : 0),
    })
    .ToList();