MySql在每個組中獲得前n個

Question

我寫了一個查詢，像：

select title, userId, created_at, deviceCode, streamTimeInSecond
from ( select title, userId, created_at, deviceCode, streamTimeInSecond,
          @userId_rank:=IF(@current_userId = userId, @userId_rank + 1, 1) as userId_rank, 
          @current_userId:=userId 
   from ViewforfirstfiveMovies order by userId, streamTimeInSecond desc ) ranked
where userId_rank<=5;

在此查詢中，我嘗試獲取數據庫中存在的每個userId查看的5個標題。 問題：對於少數用戶，我獲得了5條以上的記錄。

請幫助我解決這個問題。

Answer 1

MySQL不保證select的表達式求值順序。 因此，您只能在單個表達式中設置相互關聯的變量。

我可以這樣寫：

select title, userId, created_at, deviceCode, streamTimeInSecond
from (select title, userId, created_at, deviceCode, streamTimeInSecond,
              (@userId_rank := if(@current_userId = userId, @userId_rank + 1,
                                  if(@current_userId := userId, 1, 1)
                                 )
               ) as userId_rank,
      from ViewforfirstfiveMovies cross join
           (select @current_userId := 0, @userId_rank := 0) params
      order by userId, streamTimeInSecond desc
     ) ranked
where userId_rank <= 5;

另外，您應該在同一條語句中設置變量。