我有這樣的代碼，
IQueryable<WinnerClassExtend> dataList =
from Class in _context.AllClass
join Winner in _context.AllWinners on Class.ClassId equals Winner.ClassId
let Extend = new { Winner, Class }
group Extend by Class.ClassId into Group
select new WinnerClassExtend
{
Class = Group.Select(x => x.Class).FirstOrDefault(),
NumberOfWinnerinClass = Group.Select(x => x.Winner).Count()
};
var count = dataList.Count();
我嘗試獲取 IQuerable 的計數，就像我在 .net 框架和物體框架中所做的一樣，但似乎不起作用......并回傳錯誤“Nullable object must have a value”。
我不知道它有什么問題，我也嘗試使用它，
IQueryable<WinnerClassExtend> dataList =
_context.AllClass
.Join(_context.AllWinners , Class => Class.ClassId, Winner => Winner.ClassId, (Class, Winner) => new { Class, Winner })
.GroupBy(x => new { x.Class.ClassId })
.Select(x => new WinnerClassExtend
{
Class = x.Select(i => i.Class).FirstOrDefault(),
NumberOfWinnerinClass = x.Select(i => i.Winner).Count()
});
var count = dataList.Count();
它回傳相同的...
我已經閱讀了這個錯誤的一些問題，但它似乎與同樣的問題不同，有人可以幫忙嗎？
uj5u.com熱心網友回復：
嘗試以下查詢：
var dataList =
from Class in _context.AllClass
select new WinnerClassExtend
{
Class = Class,
NumberOfWinnerinClass = _context.AllWinners.Where(w => w.ClassId == Class.ClassId).Count()
};
var count = dataList.Count();
或者另一個：
var grouped =
from Winner in _context.AllWinners
group Winner by Class.ClassId into Group
select new
{
ClassId = Group.Key,
NumberOfWinnerinClass = Group.Count()
};
var dataList =
from Class in _context.AllClass
join g in grouped on ClassId.ClassId equals g.ClassId
select new WinnerClassExtend
{
Class = Class,
NumberOfWinnerinClass = g.NumberOfWinnerinClass
};
var count = dataList.Count();
在這兩種情況下比較執行計劃。
uj5u.com熱心網友回復：
在我看來，你需要一個左外連接：
var dataList =
from c in _context.AllClass
join w in _context.AllWinners on c.ClassId equals w.ClassId into winners
select new WinnerClassExtend
{
Class = c,
NumberOfWinnerinClass = winners.Count()
};
var count = dataList.Count();
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