SQL: How to Combine / Concatenate a Column of Results as a Single Field

I'm transferring posts from an old MovableType database to WordPress. Posts in both content management systems use "Tags" to relate posts to one another by topic or person or whatever the users want. Both systems put Posts in one table, Tags in a second table, and references between the two in yet another table -- one per row.

For my PHP  transfer program, I want to get all the Tag ID numbers that match each post. Then, I'm going to split them apart and

The table is called mt_objecttag.
The Post ID are in the column objecttag_object_id.
The Tag IDs are in the column objecttag_tag_id.

objecttag_id	objecttag_blog_id	objecttag_object_datasource	objecttag_object_id	objecttag_tag_id
37	14	entry	3066	52
36	14	entry	3066	51
34	14	entry	3067	49
35	14	entry	3067	50
1361	14	entry	3067	1142
1362	14	entry	3067	59
1363	14	entry	3067	53
27	14	entry	3068	49
28	14	entry	3068	50

I can do this using a GROUP_CONCAT function.

To make it work, I write something that loops through the ID for each Post like this:

SELECT `objecttag_object_id` AS PostID,
GROUP_CONCAT (CONVERT (objecttag_tag_id, CHAR(8) SEPARATOR '|')) AS TagIDs
FROM mt_objecttag
WHERE `objecttag_object_id`= 3067 [insert Post ID Here]
GROUP BY `objecttag_object_id`

For some reason, the Tag ID numbers will be returned as a BLOB, which is useless. So, you have to CONVERT them to a string to get useful values. (Probably, because the output is a string, you have to convert the numbers to strings to be concatenated, too.)

The SEPARATOR is optional. Using it allows you to specify the type of separator you want. Here, I specified a pipe.

PostID	TagIDs
3067	49|50|1142|59|53

Without it, you will get a comma-separated output.

PostID	TagIDs
3067	49,50,1142,59,53