我怎样才能把这些行组合起来呢?

本文关键字：组合起来 | 更新日期: 2023-09-27 18:17:03

我的Linq查询是

List<TopRated> TopRated = (from review in db.ProductReviews
                            group review by review.ProductID into product
                            select new TopRated
                            {
                              ProductName = product.Select(p => p.Products.Name).FirstOrDefault(),
                              Price = product.Select(p => p.Products.Price).FirstOrDefault(),
                              PriceOffer = product.Select(p => p.Products.PriceOffer).FirstOrDefault(),
                              ProductId = product.Key,
                              AverageRating = product.Average(p => p.Rating)
                            }).ToList();

我可以将这些行合并为一行吗?

ProductName = product.Select(p => p.Products.Name).FirstOrDefault(),
Price = product.Select(p => p.Products.Price).FirstOrDefault(),
PriceOffer = product.Select(p => p.Products.PriceOffer).FirstOrDefault(),

我认为我通过做类似于上面的事情来"添加过载"。

我怎样才能把这些行组合起来呢?

您可以使用使用范围变量的let子句。在我看来，它创建了一个直观而优雅的语法:

    var TopRated = (from review in db.ProductReviews
                    group review by review.ProductID into product
                    let firstProduct = product.FirstOrDefault()
                    select new 
                    {
                        ProductName = firstProduct.Products.Name,
                        Price = firstProduct.Products.Price,
                        PriceOffer = firstProduct.Products.PriceOffer,
                        ProductId = product.Key,
                        AverageRating = product.Average(p => p.Rating)
                    }).ToList();

要进行更深入的分析，您可以使用其中一个IL反编译器来查看每个建议的解决方案如何编译为IL。对于性能分析，您可以使用StopWatch类进行测试。

可以，在某些情况下，它会极大地影响您的查询性能。

List<TopRated> TopRated = (from review in db.ProductReviews
                        group review by review.ProductID into product
                        select new 
                        {
                          FirstProduct=product.FirstOrDefault(p=>p.Products)
                          ProductId = product.Key,
                          AverageRating = product.Average(p => p.Rating)
                        }).Select(product=>new TopRated {
                          ProductName = product.FirstProduct.Name,
                          Price = product.FirstProduct.Price,
                          PriceOffer = product.FirstProduct.PriceOffer,
                          ProductId = product.ProductId,
                          AverageRating = product.AverageRating
                        }).ToList();

你可以这样做:

List<TopRated> TopRated = 
   (from review in db.ProductReviews
    group review by review.ProductID into product
    select new { ProductId = product.Key, Products = product.FirstOrDefault().Products, AverageRating = product.Average(p => p.Rating) } into topRated
    select new TopRated
        {
            ProductName = topRated.Products.Name,
            Price = topRated.Products.Price,
            PriceOffer = topRated.Products.PriceOffer,
            ProductId = product.Key,
            AverageRating = topRated.AverageRating
        })
   .ToList();

我很想知道这对查询性能有什么影响，以及由你的方法生成的查询和由这个方法生成的查询之间的差异。

可以，但最好先进行分组和聚合，然后通过join获取产品信息，如下所示:

var query = (from review in db.ProductReviews
             group review by review.ProductID into reviews
             select new { ProductId = reviews.Key, AverageRating = reviews.Average(item => item.Rating) } into summary
             join product in db.Products on summary.ProductId equals product.Id
             select new TopRated
             {
                 ProductName = product.Name,
                 Price = product.Price,
                 PriceOffer = product.PriceOffer,
                 ProductId = product.Id,
                 AverageRating = summary.AverageRating
             });
var sqlQuery = query.ToString();
var result = query.ToList();

或者在本例中(只有一个聚合)更简单:

var query = (from product in db.Products
             select new TopRated
             {
                 ProductName = product.Name,
                 Price = product.Price,
                 PriceOffer = product.PriceOffer,
                 ProductId = product.Id,
                 AverageRating = product.ProductReviews.Average(review => review.Rating)
             });
var sqlQuery = query.ToString();
var result = query.ToList();

相关文章：

最新信息: