Question

我想在有序列表中选择用户的位置。

我只能通过将所有项目加载到内存中然后使用IndexAt()方法来查找位置来使其工作。但是，当数据库中有许多行时，这种方法效果不佳。

        public static async Task<int> GetUsersRank(DbEntities db, string userId)
    {           
      var items = await db.UserIqAnswers.Where(x => x.IqQuestion.CorrectAnswer == x.Answer).GroupBy(x => x.UserId).Select(x => new { userId = x.Key, points = x.Sum(y => y.IqQuestion.Points) })
         .OrderBy(x => x.points)
         .ToListAsync();
      return items.FindIndex(x => x.userId == userId) + 1;         
    }

我怎样才能更有效地做到这一点？

Answer 1

这样的东西？这将运行两个查询。一个用于加载当前用户的信息，另一个用于加载列表中的位置。但是，查询应该相当有效。第二个应该在SQL服务器端以COUNT（）完成。

var sumByUser = db.UserIqAnswers.Where(x => x.IqQuestion.CorrectAnswer == x.Answer)
    .GroupBy(x => x.UserId)
    .Select(x => new { userId = x.Key, points = x.Sum(y => y.IqQuestion.Points) });
var currentUser = sumByUser.Where(x => x.userId == userId).Single();
var rank = sumByUser.Where(x => x.points > currentUser.points).Count();

请注意，'sumByUser'查询永远不会被执行，它只是用作接下来两个查询的基础。

您可以使用LINQ语法将其重写为一个查询：

(from currentUser in sumByUser.Where(x => x.userId == userId)
 from rank in sumByUser.Where(x => x.points > currentUser.points
 select new { currentUser, rank = rank.Count()}).Single()

但我会检查生成的SQL以确定。

Answer 2

answer by gnud提出了正确的想法，但不幸的是两个提供的解决方案都没有很好地转换为SQL。这是因为EF6查询转换仍然对编写LINQ查询的方式很敏感（不幸的是）。

这是一个实现（通过实验找到）产生良好的翻译（将var doc = richTextBox.Document; foreach(var c in evt.Changes.Where(x => x.AddedLength > 1)) { var change = new TextRange( doc.ContentStart.GetPositionAtOffset(c.Offset), doc.ContentStart.GetPositionAtOffset(c.Offset + c.AddedLength)); Debug.WriteLine($"Change: <{change.Text}>"); }强制转换为decimal?属性类型的可空类型 - 需要强制转换以避免NRE，以防万一set为空）：

Points

生成2个这样的SQL：

public static async Task<int> GetUsersRank(DbEntities db, string userId)
{
    var userPoints = await db.UserIqAnswers
        .Where(x => x.UserId == userId && x.IqQuestion.CorrectAnswer == x.Answer)
        .SumAsync(x => (decimal?)x.IqQuestion.Points) ?? 0;

    var rank = await db.UserIqAnswers
        .Select(x => new { x.UserId, x.Answer, x.IqQuestion })
        .Where(x => x.UserId != userId && x.IqQuestion.CorrectAnswer == x.Answer)
        .GroupBy(x => x.UserId)
        .Select(x => new { userId = x.Key, points = x.Sum(y => y.IqQuestion.Points) })
        .CountAsync(x => x.points < userPoints || (x.points == userPoints && string.Compare(x.userId, userId) < 0));

    return rank;
}

和

SELECT
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT
        SUM([Extent2].[Points]) AS [A1]
        FROM  [dbo].[UserIqAnswers] AS [Extent1]
        INNER JOIN [dbo].[IqQuestions] AS [Extent2] ON ([Extent1].[Answer] = [Extent2].[CorrectAnswer]) AND ([Extent1].[IqQuestion_Id] = [Extent2].[Id])
        WHERE [Extent1].[UserId] = @p__linq__0
    )  AS [GroupBy1]

Answer 3

使用 Skip ：获取元素501 ..

queryable.Skip(500).Take(1).ToList();

实体框架在有序列表中选择项目位置

3 个答案: