EF Core 3.1 (LINQ) - 如何在多列上左加入和分组

Question

假设我有 3 个关系： classA(aID#, aName, aNumber), classB(bID#, bName, bNumber, aId), classC(cID#, cName, cNumber, bId)

并根据那 3 个带有一些随机数据的表：

A 类

aID	aName	aNumber
1	aN1	5
2	aN2	4
3	aN3	3

B类

aID	aName	aNumber	bID
1	bN1	3	1
2	bN2	4	1
3	bN3	5	2

类C

aID	aName	aNumber	bID
1	cN1	6	1
2	cN2	6	2
3	cN3	7	3

现在，使用 T-SQL 语法如下：

select a.aID, a.aName, sum(c.cNumber) as SUM
from classA a 
left join classB b on a.aID = b.aId 
left join classC c on b.bID = c.bId
group by a.AID, a.aName, c.Number

所以我得到这样的结果：

aID	aName	SUM
1	aN1	12
2	aN2	7
3	aN3	0

如何根据上面的解释创建正确的 lambda 表达式语法？

Answer 1

试试这个查询：

var query =
    from a in ctx.A
    from b in ctx.B.Where(b => a.aID == b.aId).DefaultIfEmpty()
    from c in ctx.C.Where(c => b.bID == c.bId).DefaultIfEmpty()
    group c by new { a.aID, a.aName, c.cNumber } into g
    select new 
    {
        g.Key.aID,
        g.Key.aName,
        Sum = g.Sum(x => x.cNumber)
    };

并使用 LINQ 方法链语法：

var query2 = ctx.A
    .SelectMany(a => ctx.B.Where(b => a.aID   == b.aId).DefaultIfEmpty(), (a, b) => new {a, b})
    .SelectMany(t => ctx.C.Where(c => t.b.bID == c.bId).DefaultIfEmpty(), (t, c) => new {t, c})
    .GroupBy(t => new {t.t.a.aID, t.t.a.aName, t.c.cNumber}, t => t.c)
    .Select(g => new {g.Key.aID, g.Key.aName, Sum = g.Sum(x => x.cNumber)});