具有 CASE 语句和 SUM 函数的 LINQ 查询

Question

我正在努力寻找如何使用 LINQ 查询或 LAMBDA 返回条件总和的示例。我已经独立编写了两者，但是将 CASE 与 SUM 结合起来很麻烦。我很想“作弊”并使用 SQL 视图，但我想我会先问一下。我非常感谢任何建议。这是我要转换的 SQL。

SELECT     p.product_name, 
           SUM(CASE WHEN o.order_dt <= getdate() - 1 THEN o.quantity END) AS volume_1day, 
           SUM(CASE WHEN o.order_dt <= getdate() - 7 THEN o.quantity END) AS volume_7day, 
           SUM(CASE WHEN o.order_dt <= getdate() - 30 THEN o.quantity END) AS volume_30day, 
           SUM(o.quantity) AS volume_all
FROM       products p left outer join orders o on p.product_id = o.product_id
GROUP BY   p.product_name

Answer 1

答案没有错误但不会生成优化查询。更好的答案是：

using (var context = new NorthwindEntities())
{
    DateTime volumn1Date = DateTime.Today.AddDays(-1);
    DateTime volumn7Date = DateTime.Today.AddDays(-7);
    DateTime volumn30Date = DateTime.Today.AddDays(-30);

var query = from o in context.Order_Details
                group o by o.Product.ProductName into g
                select new
                {
                    ProductName = g.Key,
                    Volume1Day = g.Sum(d => d.Order.OrderDate.Value <= volumn1Date ? (Int32?) d.Quantity : 0),
                    Volume7Day = g.Sum(d => d.Order.OrderDate.Value <= volumn7Date ? (Int32?) d.Quantity : 0),
                    Volume30Day = g.Sum(d => d.Order.OrderDate.Value <= volumn30Date ? (Int32?) d.Quantity : 0)
                };

    var list = query.ToList();
}

Answer 2

下面是一个使用 Northwinds 数据库的示例。这将为您提供您期望的结果，但 SQL 与您的示例不匹配。

using (var context = new NorthwindEntities())
{
    DateTime volumn1Date = DateTime.Today.AddDays(-1);
    DateTime volumn7Date = DateTime.Today.AddDays(-7);
    DateTime volumn30Date = DateTime.Today.AddDays(-30);

    var query = from o in context.Order_Details
                group o by o.Product.ProductName into g
                select new
                {
                    ProductName = g.Key,
                    Volume1Day = g.Where(d => d.Order.OrderDate.Value <= volumn1Date)
                                  // cast to Int32? because if no records are found the result will be a null                                              
                                  .Sum(d => (Int32?) d.Quantity),
                    Volume7Day = g.Where(d => d.Order.OrderDate.Value <= volumn7Date)
                                  .Sum(d => (Int32?) d.Quantity),
                    Volume30Day = g.Where(d => d.Order.OrderDate.Value <= volumn30Date)
                                   .Sum(d => (Int32?) d.Quantity)
                };


    var list = query.ToList();
}