MySQL多表SELECT

Question

I have three tables:

Table(attribute1, attribute2...);    
---------------------------------
Users(iduser, username)    
Link(idlink, title, userid)    
Comment(idcomment, content, linkid, userid)

How to select: Link title, with corresponding username and number of comments?

I'm currently doing like this:
Q1-Select links (SELECT * FROM `links`)
Q2-Extract usernames from previous query(Q1) - (SELECT username FROM `user` WHERE iduser=Q1.userid
Q3-Extract number of comments from Q1 by id (SELECT COUNT(*) as comments FROM `comment` WHERE linkid='Q1.idlink')

I believe we can do this in much more optimized way. I got idea how to get Link with corresponding username but I got stuck when I need to count comments.

Answer 1

SELECT iduser, username, Link.title, COUNT(idcomment)
FROM Users
LEFT JOIN Link ON (iduser = userid)
LEFT JOIN Comment ON (linkid = idlink)
GROUP BY iduser, idlink

Note that your Comment table is somewhat badly designed - the 'userid' field is not necessary, and can actually lead to situation where you've got a cross-linked record. e.g. a Comment belonging to user A might could be linked to a Link record belonging to user B.