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.

ufg_user
Key to user_id.

ufg_group
Name of the group, which the user was member of.

Schema summary
mysql> describe user_former_groups; +---+--+--+-+-+---+ +---+--+--+-+-+---+ +---+--+--+-+-+---+ 2 rows in set (0.00 sec)
 * Field    | Type             | Null | Key | Default | Extra |
 * ufg_user | int(10) unsigned | NO   | PRI | 0       |       |
 * ufg_group | varbinary(255)  | NO   | PRI |         |       |

mysql> describe user_former_groups; +---+--+--+-+-+---+ +---+--+--+-+-+---+ +---+--+--+-+-+---+
 * Field    | Type             | Null | Key | Default | Extra |
 * ufg_user | int(10) unsigned | NO   | PRI | 0       |       |
 * ufg_group | varbinary(32)   | NO   | PRI |         |       |

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 |         |       |