Manual:User former groups table

Stores the groups the user has once belonged to (will not contain groups the user had belonged to before MW 1.17). The user may again belong to these groups; check the table user_groups, if you want to know. This is used to keep MediaWiki from automatically adding users to groups from which he has been manually removed before. The table was added in.

Schema summary

 * ufg_user: Key to user_id.
 * ufg_group: Name of the group, which the user was member of.

mysql> describe user_former_groups; +---+--+--+-+-+---+ +---+--+--+-+-+---+ +---+--+--+-+-+---+ 2 rows in set (0.11 sec)
 * Field    | Type             | Null | Key | Default | Extra |
 * ufg_user | int(10) unsigned | NO   | PRI | 0       |       |
 * ufg_group | varbinary(16)   | NO   | PRI |         |       |