Question

尝试获取产品ID的列表，其中包含每个产品的反馈量需要仅计算给予产品的属于特定类别的反馈（请参阅SQL脚本：categoryId == 50）。产品可以属于多个类别。

productId, cnt
14,        0
16,        0  
15,        1
09,        2
10,        2

编辑：

我想出了下面的LINQ to SQL，重新创建我通过下面的SQL脚本表达的逻辑。但结果却不尽相同。无法从SQL脚本的LINQ逻辑中得到什么不同？

LINQ to Sql：

var result = 
(
from pcl in db.productCategoryLookup
join p in db.products on pcl.productId equals p.productId
join f in db.feedbacks on p.productId equals f.feedbackId into bb
from g in bb.DefaultIfEmpty()
where (pcl.categoryId == 50)
group p by p.productId into grp
select new
{
    productId = grp.Key,
    cnt = grp.Count()
} into res1
orderby res1.cnt
select new
{
    producetId = res1.productId,
    cnt = res1.cnt
}
)
.Take(5)
.ToList();

SQL脚本：

 SELECT TOP 5
        p.productId,
        COUNT(f.feedbackId)
    FROM ProductCategoryLookup pcl
    INNER JOIN Product p
        ON p.productId = pcl.productId
    LEFT JOIN Feedbacks f
        ON f.productId = p.productId
    WHERE 
        pcl.categoryId = 50
    GROUP BY
        p.productId
    ORDER BY
        COUNT(f.feedbackId)

表：

**Products** table
productId   PK
productName  string

**ProductCategoryLookup** table. Connects products with Category.
One product can have multiple categories and the feedback goes 
for given product in given category.
productId   FK
categoryId  FK
. . .

**Feedbacks** table. Each product+category pair gets zero or more feedbacks.
feedbackId   PK
productId    FK
categoryId   FK
 . . .

**Category** table. 
categoryId   pk
name

对于AD.Net的答案，我在'from'之后为productCategoryLookup和where子句添加了join。现在就行！感谢。

Answer 1

(from p in context.Products
select new {Product = p, Count = p.Feedbacks.Any() ? p.Feedbacks.Count() : 0})
.OrderBy(p=>p.Count)
.Take(5)
.Select(p=>p.Product)
.ToList()

Answer 2

尝试以下查询。它可能对你有帮助。

db.Products.Select(n => new { n.ProductID, count = n.Feedbacks.Count})
                .OrderBy(m=>m.count).Take(5).ToList();

无法从“group by”获得正确的输出

2 个答案: