Question

好的，这里需要一个MySQL大师。我正在尝试编写一个查询，当有人对您之前评论过的项目发表评论时，该查询将作为通知系统。 'drinkComment'表非常简单：

commentID, userID, drinkID, datetime, comment

我写了一个查询，它将获得我之前评论过的所有关于饮料的评论（这不是我的评论），但它仍会显示我评论之前发生的评论。这与我认为可行的一样接近，但事实并非如此。请帮忙！

select @drinkID:=drinkComments.drinkID, commentID, drinkID, userID, comment, datetime 
FROM drinkComments  
WHERE `drinkID` IN 
    ( select distinct drinkID from drinkComments where drinkComments.userID = 1) 
AND drinkComments.dateTime > (
/*This gets the last date user commented on the main query's drinkID*/
select datetime FROM drinkComments WHERE drinkComments.userID = 1 AND drinkComments.drinkID = @drinkID ORDER BY datetime DESC LIMIT 1
)
ORDER BY datetime DESC

Answer 1

为什么不从用户的预先查询开始，以及他们提供评论的所有饮料以及截止时间（不知道对于任何给定的饮料，您是否每人都有多条评论）。然后，在您的日期/时间评论之后找到所有其他人的评论...

这个查询实际上应该更快，因为它只是以一个用户的饮料评论作为基础开始，然后回到评论表中以查找那些与饮料ID和截止时间相匹配的信息。

SELECT STRAIGHT_JOIN
      dc.*
   from 
       ( select
               drinkID,
               max( datetime ) UserID_DrinkCommentTime
            FROM 
               drinkComments 
            WHERE
               userID = 1
            group by
               drinkID ) PreQuery
       join DrinkComments dc
         on PreQuery.DrinkID = dc.DrinkID
         and dc.datetime > PreQuery.UserID_DrinkCommentTime
   order by
      dc.DateTime desc

Answer 2

我认为你需要通过drinkID将最里面的查询与中间查询联系起来。

select @drinkID:=drinkComments.drinkID, commentID, drinkID, userID, comment, datetime 
FROM drinkComments  
WHERE `drinkID` IN 
    ( select distinct drinkID from drinkComments AS a where drinkComments.userID = 1) 
AND drinkComments.dateTime > (
/*This gets the last date user commented on the main query's drinkID*/
select datetime FROM drinkComments WHERE drinkComments.userID = 1 AND drinkComments.drinkID = a.drinkID ORDER BY datetime DESC LIMIT 1
)
ORDER BY datetime DESC

带有主查询数据变量的MySQL子查询

2 个答案: