Proper way to join tables in mysql to have a single result - php

Question

So I have a mysql database with three tables that has three table I am trying to retrieve rows of data from content based on a condition on data phpro_tag_types
The structures of the tables is like so
phpro_tag_types
tag_type_id | tag_type_name
<pk>
phpro_tag_targets
tag_target_id | tag_id | sub_tag_id | tag_target_name | tag_type_id
<pk> | <FK> | <FK> | | <FK>
content
content_id | tag_target_id | bunch of other things|
<pk> | <fk> |
The relationships between the table is like so
content.tag_target_id : phpro_tag_targets.tag_target_id
1 : m //each tag_target_id is in content once
//and phpro_tag_targets many times
phpro_tag_targets.tag_type_id : phpro_tag_types.tag_type_id
M: 1 // there is many occurrences of tag_type_id
//in phpro_tag_targets and one occurrence in of tag_type_id in phpro_tag_type
(I hope I have explained this thoroughly enough using the correct terms if not I apologize, clearly I am still kind of green with this)
Now I have a SQL query that looks like this
SELECT *
FROM phpro_tag_types types
INNER JOIN phpro_tag_targets targets ON types.tag_type_id=targets.tag_type_id
INNER JOIN content c ON targets.tag_target_id = c.tag_target_id
WHERE types.tag_type_id=14
ORDER BY update_time DESC
Now this query works however not exactly quite as I intended. The problem is the resulting array that is returned has multiple instance of the same piece of content ie. a single content_id (I believe because the same tag_target_id exists in phpro_tag_targets multiple times) however I would only like the results array to only contain unique content_id's as this is the data I am actually outputting to users.
As a side note putting Distinct into the query also doesn't seem to work as there is no way to only make sure content is DISTINCT (at least I could find)
Any help with this would be greatly appreciated as I am kind of lost on how to achieve this

Answer 1

"I am trying to retrieve rows of data from content based on a condition on data phpro_tag_types"
Assuming you are trying to fetch fields of content. What about following IN() sub-query
SELECT *
FROM content c
WHERE tag_target_id IN (
SELECT DISTINCT tag_target_id
FROM phpro_tag_types types
INNER JOIN phpro_tag_targets targets ON types.tag_type_id=targets.tag_type_id
WHERE types.tag_type_id=14
)
ORDER BY update_time DESC;
BTW,
"As a side note putting Distinct into the query also doesn't seem to work as there is no way to only make sure content is DISTINCT (at least I could find)"
DISTINCT c.* does not make sense?
SELECT DISTINCT c.*
FROM phpro_tag_types types
INNER JOIN phpro_tag_targets targets ON types.tag_type_id=targets.tag_type_id
INNER JOIN content c ON targets.tag_target_id = c.tag_target_id
WHERE types.tag_type_id=14
ORDER BY update_time DESC;

Answer 2

There's a general trick to do a left outer join on the duplicating table in such a way that only one entry has null values and then limit the query to that row:
SELECT *
FROM phpro_tag_types types
INNER JOIN phpro_tag_targets targets ON types.tag_type_id=targets.tag_type_id
INNER JOIN content c ON targets.tag_target_id = c.tag_target_id
LEFT OUTER JOIN phpro_tag_targets t2 on targets.tag_target_id=t2.tag_target_id
AND t2.tag_id < targets.tag_id
WHERE types.tag_type_id=1
AND t2.tag_target_id IS NULL;
This seems a bit crazy but does work and in my experience is performant unless you're dealing with a 'very large' data set (whatever that means).
I'm not 100% sure what the semantics here are, but this query is assuming that you don't care what 'tag_id' you get, you just want to get any tag that matches the 'type_id', which looks to me to match your intent.
NOTE: this will cause duplicate column names in the '*' select, so you need to limit to types.*, targets.*, content.* or spell out the column names.