mysql leftjoin by max autoincrement id?

Question

A) users

B) subscribtion

C) package information

I've table where B is a link between A and C , the query selected from A where B has row id for A and join C by B id .

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Full example :

SELECT a.*,c.packageTitle
FROM users AS a
LEFT JOIN subscribe AS b ON (b.userid = a.userid)
LEFT JOIN package AS c ON (b.packageid = c.packageid)

my problem if user has multi subscription in C , i cannot get latest subscription row in loop query, i also used MAX(c.packageid) inside SELECT failed also .

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Goal : get latest record in B assigned by A id .

any advice is very much appreciated

Answer 1

I don't think you were far off by trying to use MAX() to obtain the record with the latest package ID (which assumes that this ID is an increasing auto-increment column). In my answer below, the subquery identifies the latest package ID for each user record, using a GROUP BY . This subquery is then used to filter the correct records from your original query.

SELECT a.userid, b.*
FROM users AS a
LEFT JOIN subscribe AS b
    ON b.userid = a.userid
LEFT JOIN package AS c
    ON b.packageid = c.packageid
INNER JOIN
(
    SELECT a.userid, MAX(c.packageTitle) AS maxPackageTitle
    FROM users AS a
    LEFT JOIN subscribe AS b
        ON b.userid = a.userid
    LEFT JOIN package AS c
        ON b.packageid = c.packageid
    GROUP BY a.userid
) t
    ON a.userid = t.userid AND c.packageTitle = t.maxPackageTitle

As a note, this query would greatly benefit from something called a Common Table Expression (CTE), which is available in other RBDMS such as SQL Server and Oracle. A CTE would make the query much less repetitive and more readable.

Answer 2

This should be simple enough. In Mysql you just as you said put a max on the select along with a group by. So it would look something like:

SELECT username, id_info, ...
FROM
(
    SELECT a.username, a.id_info, c.packageTitle, MAX(package_id)
    FROM users AS a
    LEFT JOIN subscribe AS b
        ON b.userid = a.userid
    LEFT JOIN package AS c
        ON b.packageid = c.packageid
    GROUP BY a.username, a.id_info, c.packageTitle
)

Remember to list all columns in the select which are also being grouped, except the one on which you are taking the max, or the query will fail.