CREATE TABLE category (
  cat_id int(11) NOT NULL AUTO_INCREMENT,
  cat_name varchar(25) NOT NULL,
  PRIMARY KEY (cat_id)
)
ENGINE = INNODB
CHARACTER SET utf8
COLLATE utf8_general_ci
COMMENT = 'Категории'
ROW_FORMAT = DYNAMIC;

CREATE TABLE topic (
  t_id int(11) NOT NULL AUTO_INCREMENT,
  t_name varchar(25) NOT NULL,
  t_cat_id int(11) DEFAULT NULL,
  t_level int(11) DEFAULT NULL,
  PRIMARY KEY (t_id),
  CONSTRAINT FK_category FOREIGN KEY (t_cat_id)
  REFERENCES category (cat_id) ON DELETE RESTRICT ON UPDATE RESTRICT
)
ENGINE = INNODB
CHARACTER SET utf8
COLLATE utf8_general_ci
COMMENT = 'Темы'
ROW_FORMAT = DYNAMIC;

UPDATE topic, category   
SET topic.t_level = 555
WHERE
  topic.t_cat_id = category.cat_id AND category.cat_id = 2;