Question

我有一个用户和一个活动表。用户获得他们活动的积分，我希望在特定时间段内按积分获得前5名用户的列表：

select top 5 [firstname], [lastname], SUM(user_activities.duration) as points
from      [users]
left join [user_activities] on [users].[id] = [user_activities].[user_id] 
WHERE     [user_activities].[timestamp] >= '2018-03-01'
and       [user_activities].[timestamp] <= '2018-03-31'
group by [firstname], [lastname]
order by [points] desc, [firstname] asc

不幸的是没有活动的用户不会出现。这是因为timestamp子句的位置。有没有办法将它们包括在内？他们只有0分。

Answer 1

将WHERE子句移动到JOIN

SELECT TOP 5
            firstname
            , lastname
        , SUM(ISNULL(user_activities.duration,0)) AS points
FROM
            users
LEFT JOIN   user_activities ON  users.id = user_activities.user_id
                            AND user_activities.timestamp   >= '2018-03-01'
                            AND user_activities.timestamp   <= '2018-03-31'
GROUP BY
            firstname
            , lastname
ORDER BY
            points DESC
            , firstname ASC;

Answer 2

将WHERE切换为AND。（现在您的left join会返回inner join结果。）

即。右边的表条件应该在ON子句中！

select top 5 [firstname], [lastname], SUM(user_activities.duration) as points
from [users] left join [user_activities] on [users].[id] = [user_activities].[user_id] 
  and [user_activities].[timestamp] >= '2018-03-01'
  and [user_activities].[timestamp] <= '2018-03-31'
group by [firstname], [lastname]
order by [points] desc, [firstname] asc

在连接结果

2 个答案: