Using LIMIT in MySQL to limit results based on column value (PHP/MySQL)

Question 1

I did multiple searches for an answer to this (on SO and elsewhere), but haven't found an answer that really fit my needs (if it's out there, I apologize in advance).

I have a query, using PHP, that returns an array from a database (WordPress). Basically what I want to do is look at a column's value, and then LIMIT based on that value. Here's the array that's returned for a better idea:

http://pastebin.com/AC043qfh

In the query, you'll notice that the value for post_parent repeats for several of returned arrays. What I want to do is have it LIMIT to 3 based on the post_parent value e.g. I want 3 entries for post_parent 79, 87, 100, etc.

I'm not well versed (see: at all) in MySQL queries, but this is what I have to get that array:

SELECT DISTINCT ID, guid, post_parent, post_title 
FROM $wpdb->posts p 
WHERE p.post_type = 'attachment'
    AND p.post_mime_type LIKE 'image/%'
    AND p.post_status = 'inherit'
    AND p.post_parent IN
        (SELECT object_id FROM $term_relationships WHERE term_taxonomy_id = $post_term)

I've tried using GROUP BY, but that didn't get me what I wanted. Any help is appreciated.

EDIT Just to clarify, these are the results I want: http://pastebin.com/pWXdUuXv

Question 2

This might do the trick: (I'm assuming ID is unique, if not substitute something that is)

SELECT
  p.ID, guid, post_parent, post_title
FROM (
SELECT
  a.ID as ID,
  COUNT(*) as rank
FROM (
  SELECT ID, post_parent
  FROM $wpdb->posts
  WHERE post_type = 'attachment'
    AND post_mime_type LIKE 'image/%'
    AND post_status = 'inherit'
  ) AS a
JOIN (
  SELECT ID, post_parent
  FROM $wpdb->posts
  WHERE post_type = 'attachment'
    AND post_mime_type LIKE 'image/%'
    AND post_status = 'inherit'
  ) AS b ON b.ID <= a.ID AND b.post_parent = a.post_parent
GROUP BY a.ID
) AS r
JOIN $wpdb->posts p ON r.ID = p.ID AND r.rank <= 3
WHERE p.post_parent IN (
  SELECT object_id FROM $term_relationships
  WHERE term_taxonomy_id = $post_term)
GROUP BY p.ID
;

EDIT: Attempt to include category in rank so it'll actually work.

Specifying conditions twice is a bit ugly, but I didn't see an easy way around it.

Question 3

I would say just build it yourself. You can set it up like this:

$query = "INSERT INTO x (a,b,c) VALUES ";
foreach ($arr as $item) {
  $query .= "('".$item[0]."','".$item[1]."','".$item[2]."'),";
}
$query = rtrim($query,",");//remove the extra comma
//execute query

Don't forget to escape quotes if it's necessary.

Also, be careful that there's not too much data being sent at once. You may have to execute it in chunks instead of all at once.

Question 4

When you're gathering the file information you're overwriting $imagename on every loop so it will be assigned to the last one. Try attaching it to the $files variable (hopefully this doesn't mess with the upload class you're using).

foreach ($l as $i => $v)
{
    if (!array_key_exists($i, $files))
    $files[$i] = array();
    $files[$i][$k] = $v;
    $files[$i]['imagename'] = $_POST['keyword'][$i];
}

Then update your $sql string to reference that

$sql = 'INSERT INTO cover (id, img, keyword, category_name, cat_id) 
     VALUES ("", "'.$this_upload['image'].'", "'.$file['imagename'].'", 
         "'.$category_name.'", "'.$category.'")';

Question 5

To have all tables' statistics, you can use a UNION, with 2 or more selects, one for each table:

( SELECT s.*
       , table1.title AS name      --or whatever field you want to show
  FROM stats s
    JOIN $tableName1 table1
      ON s.id = table1.id
  WHERE tableName = '$tableName1'
)
UNION ALL
( SELECT s.*
       , table2.name AS name      --or whatever field you want to show
  FROM stats s
    JOIN $tableName2 table2
      ON s.id = table2.id
  WHERE tableName = '$tableName2'
)
UNION ALL
( SELECT s.*
       , table3.lastname AS name      --or whatever field you want to show
  FROM stats s
    JOIN $tableName3 table3
      ON s.id = table3.id
  WHERE tableName = '$tableName3'
)
;

Using Winfred's idea with LEFT JOINs. It produces different results, e.g. every field from the other tables is output in it's own column (and many NULLs occur).

SELECT s.*
     , table1.title      --or whatever fields you want to show
     , table2.name
     , table3.lastname   --etc
FROM stats s
  LEFT JOIN $tableName1 table1
    ON s.id = table1.id
      AND s.tableName = '$tableName1'
  LEFT JOIN $tableName2 table2
    ON s.id = table2.id
      AND s.tableName = '$tableName2'
  LEFT JOIN $tableName3 table3
    ON s.id = table3.id
      AND s.tableName = '$tableName3'
--this is to ensure that omited tables statistics don't appear
WHERE s.tablename IN
   ( '$tableName1'
   , '$tableName2'
   , '$tableName3'
   )
;

Question 6

just add a having clause:

select url, avg(contentping+tcpping), count(*) from websites ws, ping pi 
where ws.idwebsite = pi.idwebsite and errortype is null
group by url 
having avg(contenetping+tcpping) < 500
order by avg(contentping+tcpping) asc;

Using LIMIT in MySQL to limit results based on column value (PHP/MySQL)

Answers

php,mysql,arrays,limit