Question

这里重复两次子查询，

(select count(*) from hindi2_MOVIE as yaar where yaar.year = M.year)，我不想做，有没有办法重用它们，只需运行一次查询？

我需要找到总计数，说出按年份分组的条目“tot”，然后是其他一些功能的百分比，其中分母是总计数“tot”。在下面的SQL查询中，我重复了两次select操作，我知道这是非常昂贵的，无论如何都要重用它？

select M.year,
     (select count(*) 
      from hindi2_MOVIE as yaar 
      where yaar.year = M.year) as tot, 
    count(M.title)/(select count(*) 
                    from hindi2_MOVIE as yaar 
                    where yaar.year = M.year)*100 as perc 
from hindi2_MOVIE AS M 
where not exists (select * 
                  from hindi2_M_CAST AS C,hindi2_PERSON AS P 
                  where C.PID=P.PID AND C.MID=M.MID AND P.Gender='M') and 
                        exists(select * 
                               from hindi2_M_CAST AS C,hindi2_PERSON AS P 
                               where C.PID=P.PID AND 
                               C.MID=M.MID and Gender='F')
group by M.year;

Answer 1

您可以使用“@”符号对查询结果进行别名，并将其用于同一查询中的任何位置

select M.year, @tot:= (select count(*) from hindi2_MOVIE as yaar where
yaar.year = M.year),count(M.title)/(@tot)*100 as perc from hindi2_MOVIE AS M
where not exists (select * from hindi2_M_CAST AS C,hindi2_PERSON AS P where
C.PID=P.PID AND C.MID=M.MID AND P.Gender='M') and exists(select * from
hindi2_M_CAST AS C,hindi2_PERSON AS P where C.PID=P.PID AND C.MID=M.MID
and Gender='F') group by M.year;

Answer 2

您的查询对我来说有点混乱。但我建议你这样编写查询。这可能行不通。但请使用以下建议。

永远不要使用Count（*），使用count（字段）; 您可以通过正确使用内部联接来评估自己的差异。永远不要使用IN或Exists，使用内部连接或左连接和条件;

示例：

选择计数（M.year）为tot，计数（M.title）/计数（M.year）* 100为perc
来自hindi2_MOVIE M. 左连接hindi2_M_CAST C on（M.SomeField = C.someField）AND C.MID = M.MID和P.Gender =＆＃39; M＆＃39; - （这是不存在的）内部联接hindi2_PERSON P on C.PID = P.PID AND C.MID = M.MID and Gender =＆＃39; F＆＃39; 其中yaar.year = M.year和一些条件;

mysql中同一个表上的多个计数

2 个答案: