How to join with unknown condition like pivot in mysql - php

Question

I have the following code to
public function get_posted_questions($data) {
include dirname(__FILE__) . "/database.php";
$user_db_name = $dbconfig[$data['college_id']]['database'];
if (isset($data['start'])) {
$start = $data['start'];
} else {
$start = 0;
}
if (isset($data['end']) and ! empty($data['end'])) {
$end = $data['end'];
} else {
$end = 30;
}
/*
* multiple college database funda here goes
*/
$max_college_id = "
SELECT DISTINCT college_id
FROM just_ask_question
WHERE
status = '1'
AND isDeleted = '0'
AND college_id !='0'
ORDER BY id DESC
LIMIT $start, $end
";
$max_college_id_run = mysql_query($max_college_id);
$question_data = array();
$question_query = "
SELECT
Q.id,
Q.title,
Q.description,
Q.user_id,
Q.college_id,
Q.datetime,
IFNULL(GROUP_CONCAT(DISTINCT T.name),'') AS tags,
IFNULL(CONCAT_WS(' ',U.firstName,U.lastName),'') AS user_name
IFNULL(U.image,'') AS image,
IFNULL(V.id,'') AS no_of_view,
IFNULL(Vote.upvote,'') AS up_vote,
IFNULL(answer.id,0) AS no_of_answer,
IFNULL(is_upvote,0) AS upvote_status,
category_name
FROM just_ask_question AS Q
LEFT JOIN just_ask_question_tag Qt
ON Qt.question_id = Q.id
LEFT JOIN just_ask_tag T
ON T.id = Qt.tag_id
LEFT JOIN just_ask_category
ON just_ask_category.id = Q.category_id
LEFT JOIN
(
SELECT COUNT(id) as id, question_id FROM just_ask_answer
) AS answer ON answer.question_id = Q.id
";
while ( $row = mysql_fetch_assoc($max_college_id_run) ) {
$user_db_name = $dbconfig[$row['college_id']]['database'];
$question_query .= "
CASE WHEN Q.college_id = '".$row['college_id']."'
THEN LEFT JOIN $user_db_name.users U ON U.id = Q.user_id
";
}
$question_query .= "
LEFT JOIN
(
SELECT count(id) AS id, question_id FROM just_ask_view
) AS V ON V.question_id = Q.id
LEFT JOIN
(
SELECT
COUNT(upvote) as upvote,
question_id
FROM just_ask_upvote_downvote
WHERE upvote = '1' AND is_question = '1'
GROUP BY question_id
) AS Vote
ON Vote.question_id = Q.id
LEFT JOIN
(
SELECT
IF(COUNT(id) > 0,1,0) AS is_upvote,
question_id
FROM just_ask_upvote_downvote
WHERE
upvote = '1'
AND college_id = '" . $data['college_id'] . "'
AND user_id = '" . $data['user_id'] . "'
AND is_question = '1'
) AS Is_Vote
ON Is_Vote.question_id = Q.id
WHERE
Q.status = '1'
AND Q.isDeleted = '0'
AND CASE
WHEN Q.visibility = 0 AND Q.college_id != 0 THEN Q.college_id = '" . $data['college_id'] . "'
ELSE true
END
";
if (!empty($data['search_text'])) {
$search_text = $data['search_text'];
$question_query .= " and (Q.title like '%$search_text%' or Q.description like '%$search_text%' or T.name like '%$search_text%')";
}
$question_query .= " group by Q.id order by Q.id desc limit $start,$end";
$question_query_run = mysql_query($question_query);
/* get weather question exist */
$check_num_rows = mysql_num_rows($question_query_run);
if ($check_num_rows > 0) {
while ($row = mysql_fetch_assoc($question_query_run)) {
if ($row['image'] != '') {
$row['thumbnail'] = USER_THUMBNAIL_URL . $row['image'];
$row['image'] = IMAGE_URL . $row['image'];
} else
$row['thumbnail'] = '';
$question_data[] = $row;
}
$status['statuscode'] = "1";
$status['statusmessage'] = "ok";
$status['question_data'] = $question_data;
$response['response'] = $status;
echo json_encode($response);
die();
}else {
$status['statuscode'] = "2";
$status['statusmessage'] = "There is no record found";
$response['response'] = $status;
echo json_encode($response);
die();
}
}
AND the db config file like which contain the database name information
like
$dbconfig['1'] = array(
'host' => 'localhost',
'user_name' => 'root',
'password' => 'test123*',
'database' => 'staging_myuniversity'
);
$dbconfig['2'] = array(
'host' => 'localhost',
'user_name' => 'root',
'password' => 'test123*',
'database' => 'staging_myuniversity_dias'
);
the database are reside on same server and with full permission.
the query generate like this
select Q.id,Q.title,Q.description,Q.user_id,Q.college_id,
Q.datetime,
ifnull(group_concat(distinct T.name),'') as tags,
ifnull(CONCAT_WS(' ',
U.firstName,U.lastName),'') as user_name,
ifnull(U.image,
'') as image,ifnull(V.id,'') as no_of_view,
ifnull(Vote.upvote,
'') as up_vote,
ifnull(answer.id,0) as no_of_answer,
ifnull(is_upvote,
0
) as upvote_status,
category_name
from just_ask_question as Q
left join just_ask_question_tag Qt on Qt.question_id = Q.id
left join just_ask_tag T on T.id = Qt.tag_id
left join just_ask_category on just_ask_category.id = Q.category_id
left join
( SELECT count(id) as id,question_id
from just_ask_answer
) as answer on answer.question_id = Q.id case when Q.college_id = '1' then
left join staging_myuniversity.users U on U.id = Q.user_id case when Q.college_id = '12'then
left join campify_solutions_mathura.users U on U.id = Q.user_id case when Q.college_id = '4' then
left join staging_myuniversity_nit_kkr.users U on U.id = Q.user_id case when Q.college_id = '2' then
left join staging_myuniversity_dias.users U on U.id = Q.user_id
left join
( SELECT count(id) as id,question_id
from just_ask_view
) as V on V.question_id = Q.id
left join
( SELECT count(upvote) as upvote,question_id
from just_ask_upvote_downvote
where upvote = '1'
and is_question = '1'
group by question_id
) as Vote on Vote.question_id = Q.id
left join
( SELECT if(count(id) > 0,1,0) as is_upvote,question_id
from just_ask_upvote_downvote
where upvote = '1'
and college_id = '1'
and user_id = '1'
and is_question = '1'
) as Is_Vote on Is_Vote.question_id = Q.id
where Q.status = '1'
and Q.isDeleted = '0'
and case when Q.visibility = 0
and Q.college_id != 0 then Q.college_id = '1' else true end
group by Q.id
order by Q.id desc
limit 0,30
I need to make the join on different databases based on college id
first i fetch the distinct college id from the question then make a loop
for college id and try to make join condition but i am getting error there is any suggestion and help

Answer 1

I won't help you with entire query (I'll just list some concerns), but this "case-join" might be solved with UNION ALL subquery this way:
INNER JOIN (
SELECT 1 AS college_id, id, firstName, lastName, image
FROM staging_myuniversity.users
UNION ALL
SELECT 12 AS college_id, id, firstName, lastName, image
FROM campify_solutions_mathura.users
UNION ALL
...
) AS U ON U.college_id = Q.college_id AND U.id = Q.user_id
When you join counters with subqueries like SELECT count(), some_id use GROUP_BY some_id as well. Otherwise you'll count all rows in table and join this result for one (first/random) id only (if it passes without error).
It looks like You use LEFT JOIN too much. It means attach to the result on left regardless if any row on the right matches (joins null columns then). It's slow, gives large data sets and results with nulls for most of these columns seems useless.
Join subquery with GROUP BY clause to get concatenated tags
If large portion of this data will be repeated across the rows (see: N+1 problem) splitting queries (and merging subsets to common "header") might be a good idea.
Build your query one step at a time. Test subqueries if you're not sure of its results. Start with resource consuming parts - it might get slow at some point and you'll end up splitting it anyway.

Answer 2

... case when Q.college_id = '1' then left join ...
does not make sense. What are you trying to do?
Performance tip: Don't use LEFT JOIN ( SELECT ... ) Instead of that and IFNULL(...), simply do
SELECT ...,
IFNULL(( SELECT ... ), '') AS upvotes,
As for pivoting, I must say simply, and strongly, that some tasks are better left to your application code, not SQL.