Question

有谁能告诉我如何在Entity Framework 4.0中编写以下查询？ “博客”和“类别”是我的实体。该查询基本上返回了类别列表和该类别中的博客数量。

SELECT b.CategoryId, c.Value, Count(b.Id) AS [Count] FROM dbo.Blogs b
INNER JOIN dbo.Categories c ON b.CategoryId = c.Id
GROUP BY b.CategoryId, c.Value

提前致谢。

Answer 1

以下内容应该有效（LinqToEntities）：

var categories = from c in oc.Categories
                 select new
                 {
                     CategoryId = c.Id,
                     c.Value,
                     Count = c.Blogs.Count()
                 }

这将为您提供类别ID和值的列表，并为每个类别ID提供该类别中的博客数量。

编辑：要回答评论中的问题：在LinqToEntities中这是不可能的，但您可以在实体SQL中执行此操作。

var results = new ObjectQuery<DbDataRecord>(
    @"SELECT y, COUNT(y)
      FROM MyEntities.Blogs AS b
      GROUP BY YEAR(b.CreatedDate) AS y", entities).ToList();
var nrBlogsPerYear = from r in results
                     select new { Year = r[0], NrBlogs = r[1] };

在Entity SQL查询中，您应该将MyEntities替换为您的上下文的名称。

编辑：正如我刚刚通过Craig的评论所发现的那样，在L2E中按年份is possible进行分组，因此您可以像这样编写查询：

    var nrBlogsPerYear = from b in oc.Blogs
                         group b by b.CreatedDate.Year into g
                         select new { Year = g.Key, NrBlogs = g.Count() };

Answer 2

如果您的实体中有导航属性，则可以这样做：

var cats = from c in db.Categories
           let count = c.Blogs.Count()
           where count > 0
           select new
           {
               CategoryId = c.Id,
               Value = c.Value,
               Count = count
           };

如果您更喜欢使用显式连接，可以这样做：

var cats = from c in db.Categories
           join b in db.Blogs on c.Id equals b.CategoryId into g
           select new
           {
               CategoryId = c.Id,
               Value = c.Value,
               Count = g.Count()
           };

Answer 3

var query = from p in B.Products
            join c in B.Categories  on p.CategoryID equals c.CategoryID 
            orderby c.CategoryName,p.ProductName 
            select new 
            {
                c.CategoryName ,
                p.ProductName,
                p.ReorderLevel  
            };

GridView1.DataSource = query.ToList();
GridView1.DataBind();

实体框架4 INNER JOIN帮助

3 个答案: