text: -- MySQL
  -- create table user_subscriptions(
  --  channel_id bigint not null references usr,
  --  subscriber_id bigint not null references usr,
  --  primary key (channel_id, subscriber_id)
  -- ) engine=INNODB;
  
  -- Pg
  create table user_subscriptions (
      channel_id int8 not null references usr,
      subscriber_id int8 not null references usr,
      primary key (channel_id, subscriber_id)
  )