Writing functions

Package logic into a reusable function.

Reusable logic in the database

You can define your own functions. The simplest are SQL functions — a named, parameterized query:

CREATE FUNCTION book_count(g text) RETURNS bigint AS $$
  SELECT count(*) FROM books WHERE genre = g
$$ LANGUAGE sql;

SELECT book_count('fiction');   -- 3

The body lives between the $$ dollar quotes (so inner quotes don't need escaping). Call it anywhere an expression or table is allowed.