使用 DISTINCT(几列)和 MAX(另一列)计算行

问题描述：

我的表格包含用户对不同项目的投票.它有以下列:

My table contains votes of users for different items. It has the following columns:

id, user_id, item_id, vote, utc_time

只有 id 是唯一字段，user_id 和 utc_time 的组合可能也是唯一的.但用户可以多次为任何项目投票.

Only id is a unique field and the combination of user_id and utc_time is probably also unique. But user can cast votes for any item many times.

投票不是数字，而是具有多个可能值之一(例如，糟糕"、糟糕"、好"、极好").

A vote is not a number but rather has one of several possible values (e.g., "awful", "bad", "good", "excellent").

我需要计算有多少不同的用户为给定的#item#投了最后一票，如优秀"、良好"等.假设我只有四种不同的可能投票值，我需要获取包含以下字段的四条记录:

I need to count how many different users cast their last vote for a given #item# as "excellent", as "good", etc. So assuming I have only four different possible vote values, I need to get four records with the following fields:

vote, count_of_users

我了解如何计算所有投票，而不仅仅是用户的最后投票:

I understand how to count all votes, not only last votes of users:

SELECT vote, COUNT(id) FROM votes WHERE item_id=#item# GROUP BY vote;

但我不知道如何只计算每个用户 utc_time = MAX(utc_time) 的投票数...感谢您的帮助.

But I cannot figure out how to count only the votes where utc_time = MAX(utc_time) for each user... Thanks for your help.

这个问题与我之前的问题有关:选择一行 MAX(column) 为已知的其他几列没有子查询

This question is connected to the previous question of mine: Select one row with MAX(column) for known other several columns without subquery

答

试试这个解决方案，如果它适合你，

try this solution if it fits with you,

SELECT  a.item_ID,
        SUM(CASE WHEN a.vote = 'awful' THEN 1 ELSE 0 END) awful,
        SUM(CASE WHEN a.vote = 'bad' THEN 1 ELSE 0 END) bad,
        SUM(CASE WHEN a.vote = 'good' THEN 1 ELSE 0 END) good,
        SUM(CASE WHEN a.vote = 'excellent' THEN 1 ELSE 0 END) excellent
FROM    tableName a
        INNER JOIN
        (
            SELECT  user_ID, MAX(utc_time) max_time
            FROM    tableName
            GROUP   BY user_ID
        ) b ON a.user_ID = b.user_ID AND
                a.utc_time = b.max_time
-- WHERE   a.item_ID = 'valueHere'
GROUP   BY a.item_ID

更新 1

SELECT  a.item_ID,
        a.vote,
        COUNT(*) totalCount
FROM    tableName a
        INNER JOIN
        (
            SELECT  user_ID, MAX(utc_time) max_time
            FROM    tableName
            WHERE   item_id = 'valueHere'
            GROUP   BY user_ID
        ) b ON  a.user_ID = b.user_ID AND
                a.utc_time = b.max_time
GROUP   BY a.vote

使用 DISTINCT(几列)和 MAX(另一列)计算行

相关推荐