Linq Left Join (GroupJoin + SelectMany) Repository {id="Linq-Left-Join-GroupJoin-SelectMany-Repository"}

Repository Linq LeftJoin (GroupJoin + SelectMany) and Join

假設有兩個倉儲 A,B

要用A.bId去跟B.Id為依據

去拿B.Name欄位

最後合併 A.Id與B.Name

成為 cDto 作為資料傳輸物件輸出

Left Join (GroupJoin + Select) 子集合選擇特定欄位

var joinQuery = order
	.GroupJoin(orderStatus,
		o => o.Id,
		s => s.Id,
		(o, s) => new {
			o.Id,
			data = s.Select(x => new {
				x.Item,
				x.Qty
				}
			)
		}
	)
	.Where(x => x.data.Any())
	.ToList();

Left Join (GroupJoin + SelectMany) 攤平子集合

var joinQuery = _aRepository.GetAll()
    .GroupJoin(_bRepository.GetAll(),
    a => a.bId,
    b => b.Id,
    (a,b) => new{a,b})
    .SelectMany(result => result.b.DefaultIfEmpty(),
    (x, y) => new cDto{
        Id = x.a.Id,
        Name = y.Name ?? "N/A"
    });

Join (innerJoin)

var joinQuery = _aRepository.GetAll()
    .Join(_bRepository.GetAll(),
    a=>a.bId,
    b=>b.Id,
    (a,b)=>new cDto{
        Id = a.Id,
        Name = b.Name
    });

B倉儲只要沒有A的bID，該筆資料就不會出現在結果

參照： LINQ學習筆記(7)實作Left join(1) Join與Group join

PS5

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13 October 2025