Question

In a database structure for Question/Answer/Comment like stachexchange projects, the database schema can be three columns connected with FKs. This seems to be ideal, but I wonder what is the most efficient query to retrieve the data for a question. Answers will be simply retrieved and listed by a WHERE clause of the question_id.

My question is how to list comments nested under its corresponding answer? I am dealing with the PHP while loop to list answers and sub-lists of comments.

A very basic structure in my mind. A purpose of my question is to find idea database structure too; thus, this is merely a suggestion.

CREATE TABLE questions (
question_id int(11) NOT NULL AUTO_INCREMENT,
title varchar(255),
content text,
PRIMARY KEY (question_id);

CREATE TABLE answers (
answer_id int(11) NOT NULL AUTO_INCREMENT,
question_id int(11) REFERENCES questions(question_id),
content text,
PRIMARY KEY (answer_id);

CREATE TABLE comments (
coment_id int(11) NOT NULL AUTO_INCREMENT,
question_id int(11) REFERENCES questions(question_id),
answer_id int(11) REFERENCES answers(answer_id),
content text,
PRIMARY KEY (coment_id);

Answer 1

Assuming a Table structure as follows(simplified)

Questions Table
QID | Question

Answers Table
AID | fQID | Answer

Comments Table
CID | fAID | comment

fQID = foreign Key for Questions Table

fAID = foreign key for Answers table

Following query will select all the comments for a particular question

select comment from question join answers on QUI=fQUID join comments on AID=fAID where QID='$questionID'

Following query will select comments accourding to the Answer ID

select aid, answer, comment from question join answers on QUI=fQUID join comments on AID=fAID where AID='$answerID'

Edit:

For Nesting comments under answers, use something like this (you will need to modify this, this is very rough)

$sql = 'select comment, cid, aid, answer from question join answers on QUI=fQUID left join comments on AID=fAID where QID='$questionID' order by AID';
$result = mysql_query($sql);

$row = mysql_fetch_array($result);
echo "<div>" . $row['answer'];
$current = $row['aid'];
do{
  if($current != $row['aid']){
    print "</div>\n<div>" . $row['answer'];
    $current = $row['id'];
  }

  $current = $row['aid'];
  if($row['cid'] != null){
    print "<div>" . $row['comment'] . "</div>";
  }
}while($row = mysql_fetch_array($result));

print "</div>";