unable to join two tables, one with multiple rows - mysql

Question

I have two tables that I am attempting to join in MySQL:
reviews:
| review_id | comment | reviewer_id | user_id |
-----------------------------------------------------------
| 1 | some text. | 501 | 100 |
| 2 | lorem ipsum | 606 | 100 |
| 3 | blah blah. | 798 | 120 |
| 4 | foo bar! | 798 | 133 |
-----------------------------------------------------------
review_status:
| review_id | status | timestamp |
----------------------------------------
| 1 | 10 | 1364507521 |
| 1 | 101 | 1364508057 |
| 2 | 100 | 1364509033 |
| 1 | 150 | 1364509149 |
| 2 | 120 | 1364509283 |
| 2 | 122 | 1364855948 |
| 3 | 120 | 1364509283 |
| 3 | 122 | 1364855948 |
| 1 | 110 | 1364855945 |
| 4 | 100 | 1364509283 |
| 4 | 115 | 1364855948 |
| 4 | 210 | 1364855945 |
----------------------------------------
What I WANT is a result that looks something like this:
result
| review_id | comment | reviewer_id | user_id | status | timestamp |
--------------------------------------------------------------------------
| 1 | some text. | 501 | 100 | 200 | 1364855945 |
| 2 | lorem ipsum | 606 | 120 | 122 | 1364855948 |
--------------------------------------------------------------------------
I'm after: 1) The newest entry from the review_status table 2) A certain range of status codes (100 - 199 in this case) 3) And multiple user_id's from the review table.
This is currently my query, that I can't get to work for the life of me:
SELECT r.review_id, r.comment, r.reviewer_id, r.user_id
FROM reviews AS r
INNER JOIN
(SELECT s.status, max(s.timestamp)
FROM review_status AS s
WHERE s.status < 200
AND s.status > 99;
GROUP BY s.review_id) AS r_s
ON r.review_id = r_s.review_id
WHERE r.user_id IN (100,120);
Any help is greatly appreciated! Thanks.

Answer 1

You have a few issues with your current query.
the subquery is not returning review_id so you cannot use that in the join
you have an extra semi-colon in the subquery
I might suggest rewriting the query to use the following:
SELECT r.review_id, r.comment, r.reviewer_id, r.user_id,
rs.status, rs.timestamp
FROM reviews AS r
INNER JOIN review_status rs
ON r.review_id = rs.review_id
INNER JOIN
(
SELECT s.review_id, max(s.timestamp) MaxDate
FROM review_status AS s
WHERE s.status < 200
AND s.status > 99
GROUP BY s.review_id
) AS r_s
ON rs.review_id = r_s.review_id
AND rs.timestamp = r_s.MaxDate
WHERE r.user_id IN (100,120)
and rs.status < 200
AND rs.status > 99
See SQL Fiddle with Demo.
The main reason for the query to be written this way is because in your current query you are grouping by review_id but are returning the status. MySQL uses an extension to the GROUP BY clause that will allow items in the select list to be excluded being used in a GROUP BY or aggregate function but this could cause unexpected results. (see MySQL Extensions to GROUP BY)
From the MySQL Docs:
MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. ... You can use this feature to get better performance by avoiding unnecessary column sorting and grouping. However, this is useful primarily when all values in each nonaggregated column not named in the GROUP BY are the same for each group. The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate. Furthermore, the selection of values from each group cannot be influenced by adding an ORDER BY clause. Sorting of the result set occurs after values have been chosen, and ORDER BY does not affect which values the server chooses.

Answer 2

Try this:
SELECT r.*, r_s.*
FROM review_status r_s LEFT JOIN reviews r
ON r.review_id = r_s.review_id
WHERE r_s.user_id > 100 AND r_s.user_id < 120
ORDER BY r_s.timestamp DESC;

Answer 3

SELECT r.review_id, r.comment, r.reviewer_id, r.user_id, tt.status,tt.timestamp
FROM (
SELECT rs2.review_id,rs2.status,rs2.timestamp
FROM (
SELECT MAX(rs.timestamp) as mts
FROM reviews rr
JOIN review_status AS rs ON rs.review_id = rr.id
WHERE rs.status < 200 AND rs.status > 99
AND rr.user_id IN (100,120)
GROUP BY rs.review_id
) as t
JOIN review_status rs2 ON rs2.timestamp = t.mts
GROUP BY rs2.review_id #remove duplicate statuses with the same timestamp
) as tt
JOIN reviews as r ON r.id = tt.review_id
The user_id and status filters have to be in the innermost query to avoid selecting and join-ing the entire statuses table every time.

Answer 4

Here's my attempt with one JOIN and one correlated sub-query:
SELECT r.*, rs.*
FROM Reviews AS r
INNER JOIN Review_status AS rs ON r.review_id = rs.review_id
WHERE rs.status BETWEEN 99 AND 200 AND
r.user_id IN (100,120) AND
rs.timestamp = (SELECT MAX(timestamp) FROM Review_status
WHERE review_id = r.review_id
ORDER BY timestamp DESC)
ORDER BY r.review_id;
Its SQL Fiddle: http://sqlfiddle.com/#!2/02f18/6