We survey recent results about enumerating with constant delay the answers to a query over a database. More precisely, we focus on the case when enumeration can be achieved with a preprocessing running in time linear in the size of the database, followed by an enumeration process outputting the answers one by one with constant time between any consecutive outputs. We survey classes of databases and classes of queries for which this is possible. We also mention related problems such as computing the number of answers or sampling the set of answers.