Alas, we can't just use a Map to store the (name, value) pairs
because the output will look weird if we don't preserve the column
order. This wouldn't be a problem if we were really inserting into
a database; it only matters because we are displaying the SQL statements
via stdout instead. The LinkedHashMap class would be nice to use, but
that would require java 1.4, so we'll use a list instead, and may as
well call the entries in the list 'Column' objects.