单个查询中的 EF 多个聚合

Question

我想根据不同的条件获取一组的计数：

 var invoices = new AccountingEntities().Transactions
 var c1 = invoices.Count(i=>i.Type = 0);
 var c2 = invoices.Count(i=>i.Type = 1);
 var c3 = invoices.Count(i=>i.Type = 2);

如何在一次数据库往返中调用所有三个查询以提高性能？

Answer 1

当然，只需将您的三个计数包装在 POCO 或匿名类型中即可：

using (var invoices = new AccountingEntities())
{
    var c = (from i in invoices.Transactions
             select new 
             {
                 c1 = invoices.Count(i=>i.Type = 0),
                 c2 = invoices.Count(i=>i.Type = 1),
                 c3 = invoices.Count(i=>i.Type = 2)
             }).Single();           
}

另外，如我所示，处理您的上下文。

Answer 2

要聚合任意子查询，请使用虚拟单行结果集，从中嵌套所需的子查询。假设 db 代表您的 DbContext，计算发票类型的代码将如下所示：

var counts = (
    from unused in db.Invoices
    select new {
        Count1 = db.Invoices.Count(i => i.Type == 0),
        Count2 = db.Invoices.Count(i => i.Type == 1),
        Count3 = db.Invoices.Count(i => i.Type == 2)
    }).First();

如果想要一般性地获取所有类型的计数，请使用分组：

var counts =
    from i in db.Invoices
    group i by i.Type into g
    select new { Type = g.Key, Count = g.Count() };