Joe Celko's SQL for Smarties, 4th Edition

35

Subsets

I am defining subset operations as queries, which extract a particular subset from a given set, as opposed to set operations, which work among sets. The obvious way to extract a subset from a table is just to use a WHERE clause, which will pull out the rows that meet that criterion. But not all the subsets we want are easily defined by such a simple predicate. This chapter is a collection of tricks for constructing useful, but not obvious, subsets from a table.

35.1 Every N-th 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 ...