MYSQL使用四个LEFT JOINS和LIMIT CLAUSES显示嵌套结果

Question

I have been working with this query trying to get it to output nested results using five tables, but have failed time and time again i think I'm suppose to use sub-queries but I don't understand them enough to put them to use in my code.

I have been really beating my head can some please show me how to get the results I'm looking for.

i need to output 10 of my friends questions with 5 of their respected answers along with 1 category and 8 of the categories tags

The following is what the 10 outputs should look like:

• John Doe 1
  • Math
  • tag1 tag2, tag3 ect...
• math question answer 1
• math question answer 2
• math question answer 3
• math question answer 4
• math question answer 5
• etc...

In order to achieve the above, I'm using the following MYSQL statement and PHP snippet

<?php 

$query = $db->prepare(
    "SELECT
        question.post,          
        question.id,     
        question.user_id,

        question_responses.id response_id,
        question_responses.response,

        category.id category_id,
        category.category_name,
        category.category_posts_id cpid,

        tags.tag_name tn,
        tags.id tagid,

        friends.id myid,
        friends.logged_username,

    FROM (SELECT * FROM question ORDER BY question.id DESC LIMIT 10) AS question

    LEFT JOIN friends ON friends.friend_id = question.user_id    
    LEFT JOIN (SELECT * FROM question_responses LIMIT 5) AS question_responses ON question_responses.question_response_id = question.id
    LEFT JOIN category ON category.user_id = question.user_id
    LEFT JOIN (SELECT * FROM tags LIMIT 8) AS tags ON tags.top_tags = category.category_id

    WHERE friends.user_id = ?");

    $id = "1";

$query->bindValue(1, $id, PDO::PARAM_INT);

try {

    $query->execute();
    $questions = array();
    $question_responses = array();
    $tag = array();

    while($row = $query->fetch(PDO::FETCH_ASSOC)) {
        $question_id = $row['id'];
        $tags_id = $row['tagid'];

        $questions[$question_id] = $row;
        $tags[$tags_id][] = $row;
        $question_responses[$question_id][] = $row;
    }

    foreach ($questions as $question_id => $row) { 
        foreach ($tag as $tags_id => $row) {
            echo $row['tn'];
        }
        echo "<b>".$row['post']."</b></br></br>";
        foreach ($question_responses[$question_id] as $response_id => $row) {    
            echo $row['response']."</br></br>";
        }

    }

} catch (PDOException $e) {
    echo $e->getMessage();
    exit();
}
?>

Thanks in advance for any help.

Answer 1

Possible way to do this using lots of user variables. This is likely to be slow. It has sub queries that bring the results back with a count added, then discards the unwanted ones in the ON clause.

SELECT
    question.post,          
    question.id,     
    question.user_id,

    question_responses.id response_id,
    question_responses.response,

    category.category_id,
    category.category_name,
    category.cpid,

    category.tn,
    tagstagid,

    friends.id myid,
    friends.logged_username

FROM  friends
INNER JOIN 
(   
    SELECT category.user_id,
            category.id AS category_id,
            category.category_name,
            category.category_posts_id AS cpid,
            tags.tag_name AS tn,
            tags.id AS tagid,
            @category_user_id_cnt:=IF(@category_user_id = user_id, @category_user_id_cnt + 1, 1) AS category_user_id_cnt,
            @tag_cnt:=IF(@category_user_id = user_id, @tag_cnt + 1, 1) AS tag_cnt,
            @category_user_id := user_id
    FROM category
    CROSS JOIN (SELECT @category_user_id=0, @category_user_id_cnt:=0, @tag_cnt:=0) sub0
    LEFT OUTER JOIN tags
    ON tags.top_tags = category.category_id
) AS category
ON category.user_id = friends.user_id
AND category.category_user_id_cnt = 1
AND category.tag_cnt <= 8
LEFT JOIN
(
    SELECT post,          
            id,     
            user_id,
            @question_user_id_cnt:=IF(@question_user_id = user_id, @question_user_id_cnt + 1, 1) AS question_user_id_cnt,
            @question_user_id := user_id
    FROM
    (
        SELECT post,          
                id,     
                user_id 
        FROM question 
        ORDER BY user_id, id DESC 
    ) sub_question
    CROSS JOIN (SELECT @question_user_id=0, @question_user_id_cnt:=0) sub0
) AS question
ON friends.friend_id = question.user_id    
AND question.question_user_id_cnt <= 10
LEFT JOIN 
(
    SELECT id response_id, 
            question_responses.response,  
            @question_response_cnt:=IF(@question_response_id = question_response_id, @question_response_cnt + 1, 1) AS question_response_cnt,
            @question_response_id := question_response_id
    FROM question_responses 
    CROSS JOIN (SELECT @question_response_id=0, @question_response_cnt:=0) sub0
) AS question_responses ON question_responses.question_response_id = question.id AND question_responses.question_response_cnt <= 5

WHERE friends.user_id = ?

It might well be possible to do a faster version using tricks with GROUP_CONCAT and SUBSTRING_INDEX to have sub queries to get the latest X ids and join those back against the tables to get the rest of the data. However doing this I am struggling as I do not know if a friend can only give one answer to a question.