How to set the combination of two colums to be unique?

I have a question about DB modeling, I have a table created as following:

CREATE TABLE "users_articles" 
  ("id"         INTEGER  PRIMARY KEY  NOT NULL,
   "article_id" INTEGER,
   "user_id"    INTEGER)

1. Which statement will alter this table, so that开发者_如何学运维 the combination of article_id and user_id is unique?
2. Which statement tells me, if the DB has already been altered?

Thanks,

Markus

1. You need to create unique index:

   CREATE UNIQUE INDEX unique_users_articles ON users_articles (article_id, user_id)

2. Please clarify why you need it?
   UPDATE: Use IF NOT EXISTS to ignore index creation if it already exists:

   CREATE UNIQUE INDEX IF NOT EXISTS unique_users_articles ON users_articles (article_id, user_id)