For any query q, exists(q) builds and returns a predicate that, when executed, tests whether q produces any output. So it's a wrapper of SQL's keyword EXISTS.

q is allowed to use mappers from the enclosing context. As the PostgreSQL documentation explains:

The subquery can refer to variables from the surrounding query, which will act as constants during any one evaluation of the subquery.

I find that the most useful cases tend to take advantage of that; e.g.:

const query<movie> hall_of_fame =
    .where(exists(critics.where(critics->favorite_movie == movies->id)));

The query critics.where(critics->favorite_movie == movies->id) is not one you could execute by itself, because movies->id is not visible without the context provided by movies.where(...).

Personally I prefer to break the code up as follows:

fans_of(const exprn_mapper<serial> &movie_id) {
    return critics.where(critics->favorite_movie == movie_id);

const query<movie> hall_of_fame = movies.where(exists(fans_of(movies->id)));

The benefits are: