wiki.techinc.nl/maintenance/archives/patch-user_rights.sql

-- Split user table into two parts:
--   user
--   user_rights
-- The latter contains only the permissions of the user. This way,
-- you can store the accounts for several wikis in one central
-- database but keep user rights local to the wiki.

CREATE TABLE /*$wgDBprefix*/user_rights (
  -- Key to user_id
  ur_user int unsigned NOT NULL,

  -- Comma-separated list of permission keys
  ur_rights tinyblob NOT NULL,

  UNIQUE KEY ur_user (ur_user)

) /*$wgDBTableOptions*/;

INSERT INTO /*$wgDBprefix*/user_rights SELECT user_id,user_rights FROM /*$wgDBprefix*/user;

ALTER TABLE /*$wgDBprefix*/user DROP COLUMN user_rights;