Joe Celko's SQL for Smarties, 3rd Edition

27.1. Every nth Item in a Table

SQL is a set-oriented language, which cannot identify individual rows by their physical positions in a disk file that holds a table. Instead, a unique logical key is detected by logical expressions, and a row is retrieved. If you are given a file of employees in which the ordering of the file is based on their employee numbers, and you want to pick out every nth employee record for a survey, the job is easy. You write a procedure that loops through the file and writes every nth one to a second file.

The immediate thought of how this should be done in SQL is to simply compute MOD (emp_nbr, :n), where MOD() is the modulo function found in most SQL implementations, and save those employee rows where this function ...