第四章：约束 —— 户籍柜台不能只靠工作人员自觉

NOT NULL     -> 这栏不许空
DEFAULT      -> 没填时给默认值
UNIQUE       -> 这栏不许重号
PRIMARY KEY  -> 这份档案的总编号
CHECK        -> 这栏必须满足规则
FOREIGN KEY  -> 这份档案必须挂在另一份真实档案上

CREATE TABLE move_logs (
  move_id BIGINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
  citizen_id BIGINT UNSIGNED NOT NULL,
  from_household_id BIGINT UNSIGNED NOT NULL,
  to_household_id BIGINT UNSIGNED NOT NULL,
  moved_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
  reason VARCHAR(100) NOT NULL,
  CONSTRAINT fk_move_logs_citizen
    FOREIGN KEY (citizen_id) REFERENCES citizens(citizen_id),
  CONSTRAINT fk_move_logs_from_household
    FOREIGN KEY (from_household_id) REFERENCES households(household_id),
  CONSTRAINT fk_move_logs_to_household
    FOREIGN KEY (to_household_id) REFERENCES households(household_id),
  CONSTRAINT chk_move_diff
    CHECK (from_household_id <> to_household_id)
) ENGINE=InnoDB;