Manual:User groups table

User permissions - maps users to USER_GROUPS they are in. Users are implicitly in the "*" and "user" groups, plus any they are explicitly listed as being members of.

+--+-+--+-+-+---+ +--+-+--+-+-+---+ +--+-+--+-+-+---+
 * Field   | Type            | Null | Key | Default | Extra |
 * ug_user | int(5) unsigned | NO   | PRI | 0       |       |
 * ug_group | char(16)       | NO   | PRI | NULL    |       |

mysql> describe mw_user_groups; +--+-+--+-+-+---+ +--+-+--+-+-+---+ +--+-+--+-+-+---+ 2 rows in set (0.01 sec)
 * Field   | Type            | Null | Key | Default | Extra |
 * ug_user | int(5) unsigned | NO   | PRI | 0       |       |
 * ug_group | char(16)       | NO   | PRI | NULL    |       |