= Sql Helper

This is a small set of low-level helper methods for building and combining
standard ActiveRecord conditions of the form
    ["sql where clause", bindvar1, bindvar2, ...]

It helps you build more complex conditions by combining simpler conditions
with AND and OR. It also takes care of writing "IS NULL" conditions when
required, and can also build conditions to locate IP addresses within subnet
prefixes or vice versa.

A few simple examples:

  Sql.eq("foo", 123)
  # => ["foo=?", 123]

  Sql.eq("foo", nil)
  Sql.eq("foo", "")
  # => ["foo IS NULL"]

  data = [123, 456, 789]
  Sql.in("foo", data)
  # => ["foo IN (?,?,?)", 123, 456, 789]

  Sql.in("foo", [123, nil, 456])
  # => ["foo IN (?,?) OR foo IS NULL", 123, 456]

  Sql.and(
    Sql.eq(:foo, 123),
    ["bar=?", 456],
    ["baz=?", 789]
  )
  # => ["(foo=?) AND (bar=?) AND (baz=?)", 123, 456, 789]

  Sql.find("addr", "abc%")
  # => ["addr LIKE ?", "abc%"]

  Sql.find("addr", "192.168.0.0/30") 
  # => ["addr in (?,?,?,?,?)", "192.168.0.0/30", "192.168.0.0",
        "192.168.0.1", "192.168.0.2", "192.168.0.3"]

There are also conditions which end with a question-mark, like Sql.eq?.
These return nil (instead of a SQL condition) if the matched value is nil or
empty string, so the condition is skipped entirely. Example:

  @customers = Customer.find(:all, :conditions => Sql.and(
	Sql.find?("name", params[:name]),
	Sql.find?("postcode", params[:postcode]),
	Sql.find?("ip", params[:ip])
  ))

Here, each of the conditions may be given or omitted. If none are given, the
entire Sql.and() expression evaluates to nil, and so all customers are
found.