-- memo_relation
CREATE TABLE memo_relation (
  id INTEGER PRIMARY KEY AUTOINCREMENT,
  created_ts BIGINT NOT NULL DEFAULT (strftime('%s', 'now')),
  updated_ts BIGINT NOT NULL DEFAULT (strftime('%s', 'now')),
  memo_id INTEGER NOT NULL,
  related_memo_id INTEGER NOT NULL,
  type TEXT NOT NULL,
  description TEXT NOT NULL DEFAULT '',
  UNIQUE(memo_id, related_memo_id, type)
);