Question

我正在尝试从数据库中的每个用户（用户名）获得最近一天（review_date）写的评论（标题）

My code is :

select tb1.*, tb2.* from 
(select username, via_mobile as pc, max(review_date) as pcdate from tripadvisor where username != "" and via_mobile='false' group by username) tb1
join 
(select username, via_mobile as mobile, max(review_date) as mobile from tripadvisor whereusername != "" and via_mobile='true' group by username) tb2
on tb1.username = tb2.username;

问题是我无法在正确的日期获得正确的评论。例如：

username; review_date; title 
Alan; 2012-12-18 ;"If..."

但它应显示Alan; 2012-12-18; "Nice hotel"

你能帮我解决一下代码吗？

Answer 1

您的问题有点不清楚，但如果我理解正确，您正在查找具有最高日期的每个完整行，明确选择/按用户名分组？这应该给你：

SELECT
  username,
  review_date,
  title
FROM
  tripadvisor AS ta
  INNER JOIN (
    SELECT
      username,
      max(review_date) AS review_date
    FROM
      tripadvisor
    GROUP BY
      username
  ) AS max_table
    ON ta.username = max_table.username
      AND ta.review_date = max_table.review_date;
WHERE
  username != ''
  -- any extra where clauses go here

请参阅：How can I SELECT rows with MAX(Column value), DISTINCT by another column in SQL?

Answer 2

如果目标是返回＆＃34;移动＆＃34;的最新评论标题。对于＆＃34; pc＆＃34;，我做这样的事情：

specialIndexes.Add(longString.IndexOf(c));

如果用户只有＆＃34;移动＆＃34;评论，没有＆＃34; pc＆＃34;评论，这个查询将返回一行，但是＆＃34; pc＆＃34;列。同样，查询将为＆＃34; mobile＆＃34;返回NULL值。仅具有＆＃34; pc＆＃34;的用户的列评论

可以轻松地将查询更改为仅返回具有＆＃34; mobile＆＃34;的用户的行。和＆＃34; pc＆＃34;评论，使用INNER JOIN更接近原文。

如果目标更简单，只需返回最近的评论......

SELECT q.username
     , MAX(q.pc_date)     AS pc_date
     , MAX(p.title)       AS pc_title
     , MAX(q.mobile_date) AS mobile_date
     , MAX(r.title)       AS mobile_title
  FROM ( SELECT t.username
              , MAX(IF(t.via_mobile='false',t.review_date,NULL) AS pc_date
              , MAX(IF(t.via_mobile='true',t.review_date,NULL) AS mobile_date
           FROM tripadvisor t
          WHERE t.username <> ''
            AND t.via_mobile IN ('true','false')
          GROUP
             BY t.username
       ) q
  LEFT
  JOIN tripadvisor p
    ON p.username    = q.username
   AND p.review_date = q.pc_date
   AND p.via_mobile  = 'false'
  LEFT
  JOIN tripadvisor r
    ON r.username    = q.username
   AND r.review_date = q.mobile_date
   AND r.via_mobile  = 'true'
 GROUP
    BY q.username

当SELECT r.username , r.review_date , MAX(r.title) AS title , MAX(r.via_mobile) AS via_mobile FROM ( SELECT t.username , MAX(t.review_date) AS max_review_date FROM tripadvisor t WHERE t.username <> '' AND t.via_mobile IN ('true','false') GROUP BY t.username ) q JOIN tripadvisor r ON r.username = q.username AND r.review_date = q.max_review_date AND r.via_mobile IN ('true','false') GROUP BY r.username , r.review_date包含多个具有相同（最新）username的行时，此查询的结果有些不确定。这可以保证返回单行，但review_date和title可能不会来自同一行。

Answer 3

您的问题可以表示为存在测试：显示审核日期与该用户的最新审核日期相匹配的行。

在相关子查询中使用EXISTS测试存在性：

SELECT * FROM tripadvisor as T
where exists ( 
    select 1 from tripadvisor
    where username = T.username
    group by username
    having MAX(review_date) = T.review_date
);

选择最近一天的行

3 个答案: