Multi-tiered MySQL query? - php

Question

I have two tables(entries and tags) with a many-to-many linking table. Right now, I'm making a query to retrieve all the entries that meet my criteria, and a query for each entry to retrieve the tags.
Would there be a way to say, have the tags returned through a new column as an array in the first query?

Answer 1

This query:
SELECT e.id entry_id
, GROUP_CONCAT(t.txt ORDER BY t.txt) tags
FROM entry e
LEFT JOIN entry_tag et
ON e.id = et.entry_id
LEFT JOIN tag t
ON et.tag_id = t.id
GROUP BY e.id
Will return the tags as a comma-separated list. You can read up on GROUP_CONCAT for more options: http://dev.mysql.com/doc/refman/5.1/en/group-by-functions.html#function_group-concat
In your app you should be able to expand this into an array quite easily. For example in php, you could use explode: http://php.net/manual/en/function.explode.php
If you need more attributes from the tag or entry_tag tables, you can either add yet more GROUP_CONCAT columns, or think of some serialization format for your data (like JSON) and use GROUP_CONCAT on that, or you can simply return multiple rows per entry and process the results in the application to keep tags together with entries:
$sql = '
SELECT e.id entry_id
, t.id tag_id
, t.txt tag_text
, t.published tag_published
FROM entry e
LEFT JOIN entry_tag et
ON e.id = et.entry_id
LEFT JOIN tag t
ON et.tag_id = t.id
ORDER BY e.id
';
$result = mysql_query($ql);
$entry_id = NULL;
$entry_rows = NULL;
while ($row = mysql_fetch_assoc($result)) {
if ($entry_id != $row['entry_id']) {
if (isset($entry_id)) { //ok, found new entry
process_entry($entry_rows); //process the rows collected so far
}
$entry_id = $row['entry_id'];
$entry_rows = array();
}
$entry_rows[] = $row; //store row for his entry for later processing
}
if (isset($entry_id)){ //process the batch of rows for the last entry
process_entry($entry_rows);
}

Answer 2

You can use GROUP BY and GROUP_CONCAT function to get all tags at once as a concatenated string. http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html