text: create table wallets (
      id int not null AUTO_INCREMENT,
      version int not null,
      user_id int not null,
      amount BIGINT not null default 0,
      currency CHAR(3),
      PRIMARY KEY (id)
  );
  
  create unique index user_id_currency_idx
  on wallets(user_id, currency);
  
  create index user_id_amount_idx
  on wallets(user_id, amount);