Question

Title is bad, Hope the example will clarify this.
I have an event. Event has tags.
(Simplified) I show the event thus:

SELECT 
      *.event,
      GROUP_CONCAT(tag.name)
FROM event
JOIN tag
ON event.tag_id = tag.id
GROUP BY event.id

Say I want to filter it, and fetch only those events which have tag.id =2
But still show the entire event, with all it’s tags.

SELECT 
      *.event,
      GROUP_CONCAT(tag.name)
FROM event
JOIN tag
ON event.tag_id = tag.id
WHERE tag.id = 2
GROUP BY event.id

This won’t work, as it will show the event details correctly, but only the tag of which it’s is 2

I am pretty sure I am missing something very simple, what is it ? 🙂

Solutions I am aware of:

1. Fetch tags at a later time, after I loaded the event details.
2. Use inner query instead of the GROUP_CONCAT(tag.name) to fetch current event tags

Both seems “not clean” to me.

Answer 1

SELECT 
      event.*,
      GROUP_CONCAT(tag.name)
FROM event
  JOIN tag
    ON  event.tag_id = tag.id
  JOIN event AS event2
    ON  event2.id = event.id
    AND event2.tag_id = 2
GROUP BY event.id

or:

SELECT 
      event.*,
      GROUP_CONCAT(tag.name)
FROM event
  JOIN tag
    ON  event.tag_id = tag.id
WHERE EXISTS
      ( SELECT *
        FROM event AS event2
        WHERE event2.id = event.id
          AND event2.tag_id = 2
      )
GROUP BY event.id