Query to remove a specific secondary membergroup?

[GearTripper]

Query to remove a specific secondary membergroup from a specific usergroup; I'm thinking...

UPDATE user
REMOVE membergroupids = 'x'
WHERE usergroupid = x

Would that be correct?

[Jake Bunce]

UPDATE user
SET membergroupids = ''
WHERE usergroupid = X

This will remove all secondary group memberships for members of usergroupid X (primary group). I don't have a query to remove a single secondary group membership when the user belongs to multiple secondary groups.

[Andreas]

UPDATE user
SET membergroupids = TRIM(BOTH ',' FROM REPLACE(CONCAT(',', membergroupids, ','), ',Y,', ','))
WHERE usergroupid = X

What does this query do?
First of all, it adds a comma to the beginning and the end of membergroupids.
Afterwards, it removes ,Y,
In the final step it removes the commas added by step 1

*No Guarantee*