Question

I am creating a small message board and I am stuck

I can select the subject, the original author, the number of replies but what I can’t do is get the username, topic or date of the last post.

There are 3 tables, boards, topics and messages.

I want to get the author, date and topic of the last message in the message table. The author and date field are already fields on the messages table but i would need to join the messages and topics table on the topicid field.

this is my query that selects the subject, author, and number of replies

SELECT t.topicname, t.author, count( message ) AS message   FROM topics t   INNER JOIN messages m     ON m.topicid = t.topicid   INNER JOIN boards b     ON b.boardid = t.boardid   WHERE b.boardid = 1   GROUP BY t.topicname 

Can anyone please help me get this finished?

This is what my tables look like

CREATE TABLE `boards` (   `boardid` int(2) NOT NULL auto_increment,   `boardname` varchar(255) NOT NULL default '',   PRIMARY KEY  (`boardid`) );   CREATE TABLE `messages` (   `messageid` int(6) NOT NULL auto_increment,   `topicid` int(4) NOT NULL default '0',   `message` text NOT NULL,   `author` varchar(255) NOT NULL default '',   `date` timestamp(14) NOT NULL,   PRIMARY KEY  (`messageid`) );  CREATE TABLE `topics` (   `topicid` int(4) NOT NULL auto_increment,   `boardid` int(2) NOT NULL default '0',   `topicname` varchar(255) NOT NULL default '',   `author` varchar(255) NOT NULL default '',   PRIMARY KEY  (`topicid`) ); 

Answer 1

if your SQL supports the LIMIT clause,

SELECT m.author, m.date, t.topicname FROM messages m  JOIN topics t ON m.topicid = t.topicid  ORDER BY date desc LIMIT 1 

otherwise:

SELECT m.author, m.date, t.topicname FROM messages m  JOIN topics t ON m.topicid = t.topicid  WHERE m.date = (SELECT max(m2.date) from messages m2) 

EDIT: if you want to combine this with the original query, it has to be rewritten using subqueries to extract the message count and the date of last message:

SELECT t.topicname, t.author,        (select count(message) from messages m where m.topicid = t.topicid) AS messagecount,         lm.author, lm.date   FROM topics t   INNER JOIN messages lm     ON lm.topicid = t.topicid AND lm.date = (SELECT max(m2.date) from messages m2)   INNER JOIN boards b     ON b.boardid = t.boardid   WHERE b.boardid = 1   GROUP BY t.topicname 

also notice that if you don’t pick any field from table boards, you don’t need the last join:

SELECT t.topicname, t.author,        (select count(message) from messages m where m.topicid = t.topicid) AS messagecount,         lm.author, lm.date   FROM topics t   INNER JOIN messages lm     ON lm.topicid = t.topicid AND lm.date = (SELECT max(m2.date) from messages m2)   WHERE t.boardid = 1   GROUP BY t.topicname 

EDIT: if mysql doesn’t support subqueries in the field list, you can try this:

SELECT t.topicname, t.author, mc.messagecount, lm.author, lm.date   FROM topics t   JOIN (select m.topicid, count(*) as messagecount from messages m group by m.topicid) as mc      ON mc.topicid = t.topicid   JOIN messages lm     ON lm.topicid = t.topicid AND lm.date = (SELECT max(m2.date) from messages m2)   WHERE t.boardid = 1   GROUP BY t.topicname