Select the row for the most recent day - mysql

Question

I am trying to achieve the comment (title) written on the most recent day (review_date) from each user (username) in the database
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;
The problem is I cannot get the right review for the right date.
For example :
username; review_date; title
Alan; 2012-12-18 ;"If..."
But it should display Alan; 2012-12-18; "Nice hotel"
Can you help me to fix the code.

Answer 1

Your question is a little unclear, but if I understand correctly, you're looking for each full row with the highest date, selected distinctly/grouped by username? This should give you that:
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
See: How can I SELECT rows with MAX(Column value), DISTINCT by another column in SQL?

Answer 2

If the goal is to return the most recent review title for "mobile" and for "pc", I'd do something like this:
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
If the user has only "mobile" reviews and no "pc" reviews, this query will return a row, but with NULL values for the "pc" columns. Similarly, the query will return NULL values for the "mobile" columns for a user that has only "pc" reviews.
The query could easily be changed to only returns rows for users that have both "mobile" and "pc" reviews, to be closer to the original using the INNER JOIN.
If the goal is simpler, just to return just the most recent review...
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
The results of this query are somewhat indeterminate when a username has multiple rows with identical (most recent) review_date. This guarantees a single row will be returned, but the title and via_mobile may not be from the same row.

Answer 3

Your question could be expressed as an existence test: show the rows for which the review date matches the latest review date for that user.
Existence is tested with EXISTS in a correlated subquery:
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
);