Using GROUP_CONCAT with WHERE IN clause in MySQL

Other Questions MySQL
1

I’m having trouble with a MySQL query that combines GROUP_CONCAT and the IN operator. The subquery with GROUP_CONCAT returns 3, 5 but my main query gives me zero results even though I know those records exist in the database.

SELECT DISTINCT 
  posts.id
FROM 
  posts 
  INNER JOIN user_permissions AS perms ON 
    posts.id = perms.post_id AND posts.owner_id = perms.user_id
WHERE 
  perms.role_id IN 
  (
    SELECT 
      CAST( GROUP_CONCAT( subs.role_id ) AS CHAR )
    FROM 
      user_subscriptions AS subs
    WHERE 
      subs.external_user_id = '5'
    GROUP BY 
      subs.external_user_id
  )
  AND posts.title LIKE '%tutorial%'

I expected this to return matching posts but it’s not working as intended. What could be causing this issue?

2

classic mistake! GROUP_CONCAT creates one big string, but IN expects separate values. if you want to stick with the concat approach, try FIND_IN_SET instead - something like WHERE FIND_IN_SET(perms.role_id, (SELECT GROUP_CONCAT(subs.role_id)...)) should work better.

3

Your problem is that MySQL’s IN clause doesn’t work with concatenated strings. Your subquery returns a single string like ‘3, 5’ instead of the separate integer values that IN expects. I’ve hit this same issue before when optimizing similar queries. The fix is simple - ditch GROUP_CONCAT completely. Change your subquery to SELECT subs.role_id FROM user_subscriptions AS subs WHERE subs.external_user_id = '5' and remove the GROUP BY clause. Let the subquery return multiple rows naturally - that’s what IN is made for. Performance won’t suffer, and you’ll get correct results without the string manipulation mess.

4

You’re encountering a common issue with GROUP_CONCAT. When your subquery returns ‘3, 5’, MySQL interprets it as a single string ‘3, 5’, leading to a mismatch in your WHERE clause, which becomes perms.role_id IN (‘3, 5’). Instead, you should avoid using GROUP_CONCAT in this context since the IN clause is designed to handle multiple values from subqueries directly. Modify your subquery to: SELECT subs.role_id FROM user_subscriptions AS subs WHERE subs.external_user_id = ‘5’. This will return the individual role IDs that IN requires, and remember to remove the GROUP BY clause to get all relevant role_id values.

5

Yeah, others nailed it - GROUP_CONCAT makes a string, not individual values. But honestly? Skip rewriting the SQL and automate this whole thing instead.

I’ve been down this road with permission systems. The real issue isn’t query syntax - it’s manually managing user permissions and role mappings in raw SQL. That gets ugly quick when you scale.

Here’s what I’d do: build an automated workflow using API calls instead of hitting the database directly. Set it up to:

  1. Fetch user subscriptions via API
  2. Map roles automatically
  3. Filter posts by permissions
  4. Return clean results

You’ll dodge the SQL headaches and get a way more maintainable permission system. Plus automatic logging and error handling.

I’ve done this for several enterprise apps - works like a charm. The automation catches edge cases that manual SQL queries always miss.

Check out https://latenode.com for setting this up.