6.2.7 The #:distinct clause

Tor consistent results across all DBMSs, the query must be sorted such that rows are returned in a deterministic order, with similar rows adjacent to one another in the result set.

The #:distinct clause may be one of the following values:

• #:distinct #t dictates that rows should be completely unique - the equivalent of writing "SELECT DISTINCT foo" in SQL:

  ; (listof string)

  ; The surnames of people in the database:

  ; one result per distinct surname.

  (find-all

  (sql (select #:what     person.surname

  #:from     person

  #:distinct #t)))

• #:distinct #f completely disables uniqueness checking - the equivalent of writing SElECT foo or SELECT ALL foo in SQL:

  ; (listof string)

  ; The surnames of people in the database:

  ; one result per person.

  (find-all

  (sql (select #:what     person.surname

  #:from     person

  #:distinct #f)))

• #:distinct (expr ...) enables uniqueness checking with a custom uniqueness rule. Two results are considered equal if all exprs return the same values:

  ; (listof (list string string))

  ; The surnames of people in the database,

  ; one result per distinct surname,

  ; together with the forenames of the first

  ; person found with each surname:

  (find-all

  (sql (select #:what     (person.surname person.fornames)

  #:from     person

  #:distinct (person.surname))))