mysql complicated join - mysql

Question

I have run into some troubles while writing a query for MySQL. I don't know how to describe my problem well enough to search the web for it, so sorry if my question is stupid.
I have 3 tables:
CREATE TABLE posts( id INT, author INT );
CREATE TABLE users( id INT, nick varchar(64) );
CREATE TABLE groups( id INT, name varchar(64) );
CREATE TABLE membership (user INT, group INT, date INT ) ;
Membership contains info about users that have joined some groups. "Date" in the membership table is the time when a user joined that group.
I need a query which will return a post, its author's nick and the name of the group with the least joining date.
All I have currently is:
SELECT p.id, u.nick, g.name
FROM posts AS p
LEFT JOIN users AS u ON u.id = p.author
LEFT JOIN membership AS m ON m.user = p.author
LEFT JOIN groups AS g ON g.id = m.group
WHERE 1;
but of course it returns a random group's name, not the one with earliest joining date.
I also tried the following variant:
SELECT p.id, u.nick, g.name
FROM posts AS p
LEFT JOIN users AS u ON u.id = p.author
LEFT JOIN
(SELECT * FROM membership WHERE 1 ORDER BY date ASC)
AS m ON m.user = p.author
LEFT JOIN groups AS g ON g.id = m.group
WHERE 1;
but it gave me same result.
I would appreciate even pointers to where I could start, because at the moment I have no idea what to do with it.

Answer 1

I don't know why you want what you do, however, if you want the information for the earliest membership date (since there's no date for posting itself), no problem. Now, we have the earliest membership which will always point to the same one person as you are not asking for a specific group.. (or did you want the earliest person PER membership group -- which is what I'll write the query for). Now, we have the earliest user and can link to the posts table (by apparently the author), but what if someone has 20 posts under their name... Do you also want the FIRST ID for that author.
Just copying from your supplied tables as a reference...
posts: id (int), author(int)
users: id (int), nick (varchar)
groups: id (int), name (varchar)
membership: user (int), group (int), date (int)
select
u1.nick,
m2.date,
g1.name,
p1.id as PostID
from
( select m.group,
min( m.date ) as EarliestMembershipSignup
from
Membership m
group by
m.group ) EarliestPerGroup
join Membership m2
on EarliestPerGroup.Group = m2.Group
AND EarliestPerGroup.EarliestMembershipSignup = m2.Date
join groups g1
on m2.group = g1.id
join users u1
on m2.user = u1.ID
join posts p1
on u1.id = p1.author

Answer 2

Something like this
SELECT p.id, u.nick, g.name
FROM posts p,
users u,
membership m,
groups g
WHERE p.author = u.id
AND m.user = u.id
AND m.group = g.id
ORDER BY m.date ASC
LIMIT 1;
Take care to have good indexes when joining these 4 tables.

Answer 3

I'd recommend moving your date column from the membership table into your groups table since that seems to be where you're tracking that information. The membership table is just an intersection table for the many-to-many users<->groups tables. It should only contain user ID and the group ID columns.
What about this?
SELECT p.id, u.nick, g.name
FROM
users u,
posts p,
groups g
INNER JOIN membership m
ON u.id = m.user
INNER JOIN groups
ON m.group = groups.id
ORDER BY g.timestamp DESC
LIMIT 1;